:root {
    --bg-color: #F3F4F6;
    --board-bg: #FFFFFF;
    --board-alt-bg: #F9FAFB;

    --cell-border: #E5E7EB;
    --block-border: #9CA3AF;

    --text-given: #1F2937;
    --text-wrong: #EF4444;
    /* Modern crisp red */
    --accent: #3B82F6;
    /* Base App Theme Color */

    --text-user: var(--accent);

    --note-color: #6B7280;
    --selected-bg: color-mix(in srgb, var(--accent) 15%, var(--board-bg));

    --highlight-cell: #E5E7EB;
    /* Subtle gray for selected empty cells */
    --highlight-given: color-mix(in srgb, var(--accent) 30%, var(--board-bg));
    --highlight-user: color-mix(in srgb, var(--accent) 45%, var(--board-bg));

    --keypad-bg: #FFFFFF;
    --keypad-text: #374151;
    --keypad-shadow: 0 2px 8px rgba(0, 0, 0, 0.04);

    /* Variables for transitions */
    --transition-speed: 0.2s;
}

* {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
    -webkit-user-select: none;
    -khtml-user-select: none;
    -moz-user-select: none;
    -ms-user-select: none;
    user-select: none;
    -webkit-tap-highlight-color: transparent;
    -webkit-touch-callout: none;
    touch-action: manipulation;
}

html,
body {
    width: 100%;
    height: 100%;
    margin: 0;
    padding: 0;
    overflow: hidden;
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    overscroll-behavior: none;
    background-color: var(--bg-color);
}

body {
    font-family: 'Inter', -apple-system, BlinkMacSystemFont, sans-serif;
    background-color: var(--bg-color);
    color: var(--text-given);
    display: flex;
    justify-content: center;
    align-items: center;
}

.app-container {
    width: 100%;
    max-width: 850px;
    height: 100%;
    display: flex;
    flex-direction: column;
    background-color: var(--bg-color);
    position: relative;
    overflow: hidden;
    padding-top: env(safe-area-inset-top);
    padding-bottom: env(safe-area-inset-bottom);
}

.header {
    display: grid;
    grid-template-columns: 1fr auto 1fr;
    align-items: center;
    padding: 8px 16px;
    font-size: 16px;
    font-weight: 500;
}

.header-btn {
    background: none;
    border: none;
    font-size: 16px;
    font-weight: 500;
    font-family: inherit;
    cursor: pointer;
    color: var(--accent);
    transition: opacity var(--transition-speed);
    justify-self: start;
}

.bottom-btn {
    background: none;
    border: none;
    font-size: 16px;
    font-weight: 500;
    font-family: inherit;
    cursor: pointer;
    color: var(--accent);
    transition: opacity var(--transition-speed);
}

@media (hover: hover) {

    .header-btn:hover,
    .bottom-btn:hover {
        opacity: 0.7;
    }
}

.difficulty {
    color: #6B7280;
    font-weight: 500;
    justify-self: center;
}

.timer {
    color: #6B7280;
    font-weight: 500;
    justify-self: end;
    font-variant-numeric: tabular-nums;
}

.board-container {
    flex-grow: 1;
    display: flex;
    align-items: center;
    justify-content: center;
    padding: 0 4px;
    width: 100%;
    min-height: 0;
    overflow: auto;
}

.board-container.is-zoomed {
    align-items: flex-start;
    justify-content: flex-start;
}

.board-wrapper {
    /* Pure algebraic lock taking out max-width and max-height bounds to completely eliminate Safari flexbox conflicts. */
    width: min(calc(100vw - 8px), calc(100dvh - 350px - env(safe-area-inset-top) - env(safe-area-inset-bottom)), 768px);
    height: min(calc(100vw - 8px), calc(100dvh - 350px - env(safe-area-inset-top) - env(safe-area-inset-bottom)), 768px);
    aspect-ratio: 1;
    background-color: var(--board-bg);
    border-radius: 12px;
    box-shadow: 0 8px 20px rgba(0, 0, 0, 0.05);
    padding: 6px;
    flex-shrink: 0;
    transition: zoom 0.2s ease, width 0.2s ease, height 0.2s ease;
}

.board-container.is-zoomed .board-wrapper {
    width: calc(1.5 * min(calc(100vw - 8px), calc(100dvh - 350px - env(safe-area-inset-top) - env(safe-area-inset-bottom)), 768px));
    height: calc(1.5 * min(calc(100vw - 8px), calc(100dvh - 350px - env(safe-area-inset-top) - env(safe-area-inset-bottom)), 768px));
    margin: 24px auto;
}

.board {
    width: 100%;
    height: 100%;
    display: grid;
    grid-template-columns: repeat(9, minmax(0, 1fr));
    grid-template-rows: repeat(9, minmax(0, 1fr));
    background-color: var(--block-border);
    border: 2px solid var(--block-border);
    border-radius: 4px;
    overflow: hidden;
    gap: 0;
}

.cell {
    background-color: var(--board-bg);
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: clamp(20px, min(8vw, 6vh), 36px);
    font-weight: 500;
    color: var(--text-given);
    position: relative;
    border-right: 1px solid var(--cell-border);
    border-bottom: 1px solid var(--cell-border);
    cursor: pointer;
    transition: background-color 0.1s;
}

.cell.alt-bg {
    background-color: var(--board-alt-bg);
}

.cell.thick-right {
    border-right: 2px solid var(--block-border);
}

.cell.thick-bottom {
    border-bottom: 2px solid var(--block-border);
}

/* User Answers */
.cell.user-answer {
    color: var(--text-user);
    font-weight: 600;
}

/* Wrong Answers */
.cell.wrong-answer {
    color: var(--text-wrong) !important;
    font-weight: 700;
}

/* Selected Cell (empty) */
.cell.selected-empty {
    background-color: var(--selected-bg) !important;
    box-shadow: inset 0 0 0 2px var(--accent);
}

/* Highlighted Given Number */
.cell.highlighted-given {
    background-color: var(--highlight-given) !important;
}

/* Highlighted User Number */
.cell.highlighted-user {
    background-color: var(--highlight-user) !important;
}

/* Selected Given Number Focus */
.cell.selected-given-focus {
    background-color: var(--accent) !important;
    color: white !important;
}

/* If the focused cell is a wrong answer, it should be immediately red */
.cell.wrong-answer.selected-given-focus {
    background-color: #FEE2E2 !important;
    color: var(--text-wrong) !important;
    box-shadow: inset 0 0 0 2px var(--text-wrong);
}

/* Notes Grid */
.notes-grid {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    grid-template-rows: repeat(3, 1fr);
    width: 100%;
    height: 100%;
    position: absolute;
    top: 0;
    left: 0;
    padding: 2px;
}

.note {
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: clamp(9px, min(2.5vw, 2vh), 14px);
    color: var(--note-color);
    font-weight: 600;
    line-height: 1;
}

.note.highlighted-note {
    color: #FFFFFF !important;
    background-color: var(--accent) !important;
    border-radius: 50%;
    font-weight: 700 !important;
    transform: scale(1.1) !important;
}

body.dark-mode .note.highlighted-note {
    color: #FFFFFF !important;
    background-color: color-mix(in srgb, var(--accent) 55%, transparent) !important;
    font-weight: 800 !important;
    transform: scale(1.2) !important;
    border-radius: 50% !important;
}

.board-container.is-zoomed .notes-grid {
    padding: 3px;
}

.board-container.is-zoomed .note {
    font-size: clamp(14px, min(3.8vw, 3.2vh), 21px);
}

/* Keypad */
.keypad-container {
    padding: 8px 16px;
    background-color: var(--bg-color);
}

.keypad {
    display: grid;
    grid-template-columns: repeat(5, 1fr);
    grid-template-rows: repeat(2, 1fr);
    gap: 6px;
}

.key {
    background-color: var(--keypad-bg);
    border: none;
    border-radius: 12px;
    color: var(--keypad-text);
    font-size: clamp(34px, 10vw, 44px);
    font-family: inherit;
    font-weight: 500;
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: space-between;
    padding: 10px 0 6px 0;
    aspect-ratio: 0.65;
    position: relative;
    cursor: pointer;
    box-shadow: var(--keypad-shadow);
    transition: transform var(--transition-speed), background-color var(--transition-speed), color var(--transition-speed);
}

.key span {
    margin-top: auto;
    line-height: 1.1;
    width: 100%;
    border-top: 2px dotted #D1D5DB;
    padding-top: 8px;
}

.key:active {
    transform: scale(0.95);
}

.key.selected-key {
    background-color: var(--accent);
    color: white;
    box-shadow: 0 4px 12px rgba(59, 130, 246, 0.3);
}

/* Note Mode Affordances */
body.note-mode .keypad .key:not(.pencil-key) {
    box-shadow: none;
    background-color: transparent;
    border: 2px dashed #9CA3AF;
    color: #6B7280;
}

body.note-mode .keypad .key:not(.pencil-key) span {
    font-size: clamp(34px, 8vw, 44px);
    font-weight: 600;
    border-top: none;
    padding-top: 0;
}

body.note-mode .keypad .key.selected-key:not(.pencil-key) {
    background-color: rgba(59, 130, 246, 0.15) !important;
    border-color: var(--accent) !important;
    color: var(--accent) !important;
    box-shadow: none !important;
}

.mini-grid {
    position: relative;
    top: auto;
    left: auto;
    transform: none;
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    grid-template-rows: repeat(3, 1fr);
    gap: 3px;
    width: 32px;
    height: 32px;
    padding: 8px;
    border-radius: 8px;
    cursor: pointer;
    z-index: 5;
    transition: background-color var(--transition-speed), transform var(--transition-speed);
}

.highlight-bubble:hover {
    background-color: rgba(107, 114, 128, 0.1);
}

.highlight-bubble:active {
    transform: scale(0.85);
}

.mini-grid div {
    background-color: currentColor;
    border-radius: 50%;
    opacity: 0.15;
}

.mini-grid div.filled {
    opacity: 0.9;
}

.pencil-key {
    position: relative;
    color: var(--accent);
    flex-direction: column;
}

.pencil-key.pencil-active {
    background-color: var(--accent);
    color: white;
}

.pencil-key.pencil-active svg {
    fill: white;
}

.pencil-key span {
    border-top: none;
}

.pencil-key svg {
    fill: var(--accent);
    width: 44px;
    height: 44px;
    margin-bottom: 8px;
    transition: fill var(--transition-speed), transform 0.2s;
}

#pencil-badge {
    position: absolute;
    bottom: 8px;
    font-size: 11px;
    font-weight: 700;
    letter-spacing: 0px;
    color: inherit;
}

.bottom-bar {
    display: flex;
    justify-content: space-between;
    padding: 2px 16px;
    padding-bottom: 17px;
}

.bottom-left,
.bottom-right,
.bottom-center {
    display: flex;
    gap: 24px;
}

#zoom-btn {
    color: var(--accent);
}

.bottom-btn.disabled {
    color: #9CA3AF;
    cursor: default;
}

.bottom-btn.disabled:hover {
    opacity: 1;
}

/* Ad Banner */
.ad-banner-placeholder {
    width: 100%;
    height: calc(75px + env(safe-area-inset-bottom));
    padding-bottom: env(safe-area-inset-bottom);
    margin-bottom: calc(-1 * env(safe-area-inset-bottom));
    /* Scaled mobile banner height constraint */
    background-color: #E5E7EB;
    display: flex;
    align-items: center;
    justify-content: center;
    color: #9CA3AF;
    font-size: 12px;
    font-weight: 500;
    text-transform: uppercase;
    letter-spacing: 0.05em;
    flex-shrink: 0;
    z-index: 50;
    /* Ensure ad acts as overlay if needed on tiny screens */
}

/* Modal Overlay for Menu */
.modal-overlay {
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background: rgba(0, 0, 0, 0.4);
    backdrop-filter: blur(2px);
    display: flex;
    justify-content: center;
    align-items: center;
    z-index: 100;
    opacity: 0;
    pointer-events: none;
    transition: opacity var(--transition-speed);
}

.modal-overlay.active {
    opacity: 1;
    pointer-events: auto;
}

/* Pause Overlay State */
body.paused .header,
body.paused .board-container,
body.paused .keypad-container,
body.paused .bottom-bar,
body.locked .header,
body.locked .board-container,
body.locked .keypad-container,
body.locked .bottom-bar {
    filter: blur(8px);
    opacity: 0.5;
    pointer-events: none;
    transition: filter var(--transition-speed), opacity var(--transition-speed);
}

.pause-overlay {
    position: absolute;
    top: 60px;
    /* Below Header */
    bottom: 90px;
    /* Above Ad Banner */
    left: 0;
    right: 0;
    z-index: 40;
    display: flex;
    justify-content: center;
    align-items: center;
    opacity: 0;
    pointer-events: none;
    transition: opacity var(--transition-speed);
}

body.paused .pause-overlay {
    opacity: 1;
    pointer-events: auto;
}

.pause-content {
    text-align: center;
    display: flex;
    flex-direction: column;
    align-items: center;
}

.pause-content h2 {
    font-size: 28px;
    font-weight: 700;
    color: var(--text-given);
    margin-bottom: 8px;
}

.pause-content p {
    font-size: 14px;
    color: #6B7280;
    margin-bottom: 24px;
}

.resume-large-btn {
    background-color: var(--accent);
    color: white;
    border: none;
    border-radius: 24px;
    padding: 14px 32px;
    font-size: 18px;
    font-weight: 600;
    cursor: pointer;
    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.1);
    transition: transform var(--transition-speed);
}

.resume-large-btn:active {
    transform: scale(0.95);
}

.menu-modal {
    background: #FFFFFF;
    width: 90%;
    max-width: 320px;
    border-radius: 16px;
    display: flex;
    flex-direction: column;
    overflow: hidden;
    box-shadow: 0 20px 25px -5px rgba(0, 0, 0, 0.1), 0 10px 10px -5px rgba(0, 0, 0, 0.04);
}

.menu-item {
    padding: 18px;
    text-align: center;
    font-size: 16px;
    font-weight: 500;
    color: var(--accent);
    border-bottom: 1px solid #F3F4F6;
    background: transparent;
    border-top: none;
    border-left: none;
    border-right: none;
    cursor: pointer;
    transition: background-color 0.1s;
}

.menu-item:hover {
    background-color: #F9FAFB;
}

.menu-title {
    color: #9CA3AF;
    font-size: 14px;
    font-weight: 600;
    text-transform: uppercase;
    letter-spacing: 0.05em;
    padding: 20px 18px 10px;
    text-align: center;
    border-bottom: 1px solid #F3F4F6;
    cursor: default;
}

.menu-cancel {
    background: #F9FAFB;
    font-weight: 600;
    border-bottom: none;
    color: #4B5563;
}

/* Settings Modal Specifics */
.settings-row {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 18px;
    border-bottom: 1px solid #F3F4F6;
    color: var(--text-given);
    font-size: 16px;
    font-weight: 500;
}

.section-divider {
    border-top: 8px solid #F3F4F6;
}

/* Toggle Switch */
.toggle-switch {
    position: relative;
    display: inline-block;
    width: 44px;
    height: 24px;
}

.toggle-switch input {
    opacity: 0;
    width: 0;
    height: 0;
}

.slider {
    position: absolute;
    cursor: pointer;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background-color: #D1D5DB;
    transition: .4s;
    border-radius: 24px;
}

.slider:before {
    position: absolute;
    content: "";
    height: 18px;
    width: 18px;
    left: 3px;
    bottom: 3px;
    background-color: white;
    transition: .4s;
    border-radius: 50%;
}

input:checked+.slider {
    background-color: var(--accent);
}

input:checked+.slider:before {
    transform: translateX(20px);
}

/* Themes */
.theme-options {
    display: flex;
    gap: 10px;
}

.theme-btn {
    width: 24px;
    height: 24px;
    border-radius: 50%;
    border: 2px solid transparent;
    cursor: pointer;
    transition: transform 0.2s;
}

.theme-btn.active {
    transform: scale(1.2);
    border-color: var(--text-given);
}

.theme-blue {
    background-color: #3B82F6;
}

.theme-cyan {
    background-color: #06B6D4;
}

.theme-green {
    background-color: #10B981;
}

.theme-orange {
    background-color: #F97316;
}

.theme-amber {
    background-color: #D97706;
}

.theme-purple {
    background-color: #8B5CF6;
}

body.dark-mode {
    --bg-color: #111827;
    --board-bg: #1F2937;
    --board-alt-bg: #111827;

    --cell-border: #374151;
    --block-border: #6B7280;

    --text-given: #D1D5DB;
    /* Noticeably softer gray to push it to the background */
    --text-user: color-mix(in srgb, var(--accent) 60%, white);
    /* Very bright neon-pastel to pop in the dark! */
    --text-wrong: #FCA5A5;

    --note-color: #E5E7EB;

    --selected-bg: color-mix(in srgb, var(--accent) 20%, transparent);
    --highlight-given: color-mix(in srgb, var(--accent) 65%, transparent);
    --highlight-user: color-mix(in srgb, var(--accent) 85%, transparent);

    --keypad-bg: #374151;
    --keypad-text: #F3F4F6;
    --keypad-shadow: 0 4px 12px rgba(0, 0, 0, 0.4);
    --keypad-border: #4B5563;
}

body.dark-mode .key span {
    border-top-color: #6B7280;
}

body.dark-mode .menu-modal {
    background: #1F2937;
}

body.dark-mode .ad-banner-placeholder {
    background-color: #374151;
    color: #6B7280;
}

body.dark-mode.note-mode .keypad .key:not(.pencil-key) {
    border: 2px dashed #4B5563;
    color: #9CA3AF;
}

body.dark-mode.note-mode .keypad .key.selected-key:not(.pencil-key) {
    color: #60A5FA !important;
}

body.dark-mode .settings-row {
    border-bottom: 1px solid #374151;
}

body.dark-mode .section-divider {
    border-top-color: #111827;
}

body.dark-mode .theme-btn.active {
    border-color: #F9FAFB;
}

body.dark-mode .menu-item {
    border-bottom: 1px solid #374151;
}

body.dark-mode .menu-item:hover {
    background-color: #111827;
}

body.dark-mode .menu-title {
    border-bottom: 1px solid #374151;
}

body.dark-mode .menu-cancel {
    background: #374151;
    color: #F9FAFB;
}

body.dark-mode .cell.wrong-answer.selected-given-focus {
    background-color: #7F1D1D !important;
}

body.dark-mode .bottom-btn.disabled {
    color: #4B5563;
}

body.dark-mode .pencil-key {
    color: #F9FAFB;
}

body.dark-mode .pencil-key svg {
    fill: #F9FAFB;
}

body.dark-mode .pencil-key.pencil-active {
    background-color: #F9FAFB;
    color: var(--board-bg);
}

body.dark-mode .pencil-key.pencil-active svg {
    fill: var(--board-bg);
}

@media (min-width: 768px) {
    .app-container {
        max-width: 100vw;
    }

    .header {
        padding: 16px 32px;
    }

    .header-btn,
    .bottom-btn,
    .difficulty,
    .timer {
        font-size: 26px;
    }

    .board-wrapper {
        width: min(calc(100vw - 48px), calc(100dvh - 350px - env(safe-area-inset-top) - env(safe-area-inset-bottom)), 900px);
        height: min(calc(100vw - 48px), calc(100dvh - 350px - env(safe-area-inset-top) - env(safe-area-inset-bottom)), 900px);
    }

    .board-container.is-zoomed .board-wrapper {
        width: calc(1.5 * min(calc(100vw - 48px), calc(100dvh - 350px - env(safe-area-inset-top) - env(safe-area-inset-bottom)), 900px));
        height: calc(1.5 * min(calc(100vw - 48px), calc(100dvh - 350px - env(safe-area-inset-top) - env(safe-area-inset-bottom)), 900px));
    }

    .keypad-container {
        padding-top: 24px;
        padding-bottom: 0px;
    }

    .keypad {
        grid-template-columns: repeat(10, 1fr);
        grid-template-rows: 1fr;
        gap: 12px;
        padding: 0 16px;
    }

    .key {
        aspect-ratio: 0.6;
        padding: 16px 0 0 0;
    }

    .key span {
        height: 50%;
        margin-top: auto;
        display: flex;
        align-items: center;
        justify-content: center;
        border-top: 2px dotted #D1D5DB;
        padding-top: 0;
    }

    body.dark-mode .key span {
        border-top-color: #6B7280;
    }

    body.note-mode .keypad .key:not(.pencil-key) span {
        height: auto;
        display: block;
    }

    .zoom-text-ipad {
        display: block !important;
        font-size: 26px;
        font-weight: 500;
    }

    #zoom-icon {
        width: 38px;
        height: 38px;
    }

    .mini-grid {
        width: 46px;
        /* Greatly enlarged for iPad numbers */
        height: 46px;
        gap: 4px;
        /* Slightly thicker spacing */
    }

    .bottom-bar {
        padding-top: 24px;
        padding-bottom: 0px;
        /* Adjusted to equalise bottom gap */
        padding-left: 24px;
        padding-right: 24px;
    }

    .ad-banner-placeholder {
        margin-top: 24px;
    }

    .pencil-key svg {
        width: 60px;
        height: 60px;
        margin-bottom: 16px;
    }

    #pencil-badge {
        font-size: 16px;
        bottom: 14px;
    }

    .note {
        font-size: 20px;
    }

    .board-container.is-zoomed .note {
        font-size: 32px;
    }

    .menu-modal {
        max-width: 480px;
        border-radius: 20px;
    }

    .menu-item,
    .settings-row {
        padding: 24px;
        font-size: 20px;
    }

    .menu-title {
        font-size: 18px;
        padding: 26px 24px 14px;
    }

    .theme-options {
        gap: 16px;
    }

    .theme-btn {
        width: 34px;
        height: 34px;
        border-width: 3px;
    }

    .toggle-switch {
        width: 60px;
        height: 32px;
    }

    .slider {
        border-radius: 32px;
    }

    .slider:before {
        height: 24px;
        width: 24px;
        left: 4px;
        bottom: 4px;
    }

    input:checked+.slider:before {
        transform: translateX(28px);
    }
}

@keyframes winFlip {
    0% {
        filter: brightness(1);
        transform: perspective(400px) rotateY(0deg);
        z-index: 1;
    }

    50% {
        filter: brightness(1.2);
        transform: perspective(400px) rotateY(180deg) scale(1.15);
        box-shadow: 0 4px 15px var(--accent);
        background-color: var(--accent);
        color: white;
        border-radius: 6px;
        z-index: 10;
    }

    100% {
        filter: brightness(1);
        transform: perspective(400px) rotateY(360deg) scale(1);
        z-index: 1;
    }
}