:root {
    /* Base Variables (Light Mode Default) */
    --bg-gradient: linear-gradient(135deg, #f5f7fa 0%, #c3cfe2 100%);
    --text-color: #2d3436;
    --primary-color: #6c5ce7;
    --primary-hover: #5b4cc4;
    --secondary-color: rgba(255, 255, 255, 0.6);
    --secondary-hover: rgba(255, 255, 255, 0.8);
    --glass-bg: rgba(255, 255, 255, 0.65);
    --glass-border: rgba(255, 255, 255, 0.4);
    --board-border: rgba(45, 52, 54, 0.6);
    /* Darker, more visible */
    --cell-border: rgba(45, 52, 54, 0.2);
    --highlight-bg: rgba(108, 92, 231, 0.15);
    --hover-bg: rgba(255, 255, 255, 0.5);
    --fixed-num-color: #2d3436;
    --user-num-color: #0984e3;
    --selected-bg: #6c5ce7af;
    --selected-text: #ffffff;
    --shadow-sm: 0 4px 6px rgba(0, 0, 0, 0.05);
    --shadow-md: 0 8px 32px rgba(31, 38, 135, 0.15);

    --transition: all 0.3s cubic-bezier(0.25, 0.8, 0.25, 1);
}

* {
    box-sizing: border-box;
}

[data-theme="dark"] {
    --bg-gradient: linear-gradient(135deg, #1e1e2e 0%, #2d3436 100%);
    --text-color: #dfe6e9;
    --primary-color: #a29bfe;
    --primary-hover: #8e44ad;
    --secondary-color: rgba(0, 0, 0, 0.3);
    --secondary-hover: rgba(0, 0, 0, 0.5);
    --glass-bg: rgba(30, 30, 46, 0.75);
    --glass-border: rgba(255, 255, 255, 0.1);
    --board-border: rgba(223, 230, 233, 0.4);
    --cell-border: rgba(223, 230, 233, 0.15);
    --highlight-bg: rgba(162, 155, 254, 0.15);
    --hover-bg: rgba(255, 255, 255, 0.05);
    --fixed-num-color: #dfe6e9;
    --user-num-color: #74b9ff;
    --selected-bg: #a29bfe;
    --selected-text: #1e1e2e;
    --shadow-sm: 0 4px 6px rgba(0, 0, 0, 0.2);
    --shadow-md: 0 8px 32px rgba(0, 0, 0, 0.4);
}

body {
    font-family: 'Outfit', sans-serif;
    background: var(--bg-gradient);
    color: var(--text-color);
    margin: 0;
    display: flex;
    justify-content: center;
    min-height: 100vh;
    transition: var(--transition);
    overflow-x: hidden;
}

.container {
    width: 100%;
    max-width: 550px;
    padding: 30px 20px;
    display: flex;
    flex-direction: column;
    align-items: center;
}

header {
    width: 100%;
    display: flex;
    flex-direction: column;
    align-items: center;
    margin-bottom: 20px;
    margin-bottom: 20px;
}

.header-top-row {
    width: 100%;
    display: grid;
    grid-template-columns: 1fr auto 1fr;
    align-items: center;
    margin-bottom: 20px;
    padding: 0 10px;
}

.theme-toggle {
    justify-self: end;
}

.left-controls {
    display: flex;
    gap: 10px;
}

.icon-toggle {
    background: var(--secondary-color);
    border: none;
    border-radius: 50%;
    width: 40px;
    height: 40px;
    cursor: pointer;
    font-size: 1.2rem;
    display: flex;
    align-items: center;
    justify-content: center;
    transition: var(--transition);
    color: var(--text-color);
    box-shadow: var(--shadow-sm);
    backdrop-filter: blur(4px);
}

h1 {
    font-weight: 200;
    font-size: 2.2rem;
    margin: 0;
    letter-spacing: 4px;
    text-transform: uppercase;
    text-shadow: 0 2px 4px rgba(0, 0, 0, 0.1);
    text-align: center;
}

.header-controls {
    display: flex;
    gap: 12px;
    align-items: center;
    justify-content: center;
    flex-wrap: wrap;
    width: 100%;
    padding: 12px 16px;
    box-sizing: border-box;
}

.control-group {
    display: flex;
    gap: 8px;
    align-items: center;
}

.stats-group {
    display: flex;
    gap: 16px;
    align-items: center;
    padding: 8px 16px;
    background: var(--secondary-color);
    border-radius: 12px;
    border: 1px solid var(--glass-border);
}

.stat-item {
    display: flex;
    align-items: center;
    gap: 6px;
    font-size: 1.1rem;
    font-weight: 600;
    font-variant-numeric: tabular-nums;
    color: var(--text-color);
}

.stat-item svg {
    opacity: 0.7;
}

.stat-mistakes {
    color: #e17055;
}

[data-theme="dark"] .stat-mistakes {
    color: #ff7675;
}

/* Icon Buttons */
.btn-icon-primary,
.btn-icon-secondary {
    display: flex;
    align-items: center;
    justify-content: center;
    width: 40px;
    height: 40px;
    border: none;
    border-radius: 10px;
    cursor: pointer;
    transition: var(--transition);
    backdrop-filter: blur(4px);
}

.btn-icon-primary {
    background: var(--primary-color);
    color: white;
    box-shadow: 0 4px 12px rgba(108, 92, 231, 0.3);
}

.btn-icon-secondary {
    background: var(--secondary-color);
    color: var(--text-color);
    border: 1px solid var(--glass-border);
}

@media (hover: hover) {
    .btn-icon-primary:hover {
        transform: translateY(-2px) scale(1.05);
        box-shadow: 0 6px 16px rgba(108, 92, 231, 0.4);
    }

    .btn-icon-secondary:hover {
        background: var(--primary-color);
        color: white;
        transform: translateY(-2px);
        border-color: transparent;
    }
}

.btn-icon-primary:active,
.btn-icon-secondary:active {
    transform: translateY(0);
}

/* Middle Controls */
.controls-middle {
    display: flex;
    gap: 15px;
    margin-bottom: 30px;
    width: 100%;
    justify-content: center;
}

.controls-middle button {
    font-size: 1.5rem;
    padding: 8px 0;
    width: 60px;
    display: flex;
    justify-content: center;
    align-items: center;
}

/* Glassmorphism Cards/Elements */
.glass-panel {
    background: var(--glass-bg);
    backdrop-filter: blur(12px);
    -webkit-backdrop-filter: blur(12px);
    border: 1px solid var(--glass-border);
    border-radius: 16px;
    box-shadow: var(--shadow-md);
}

select {
    padding: 10px 16px;
    border-radius: 12px;
    border: 1px solid var(--glass-border);
    font-family: inherit;
    background: var(--secondary-color);
    color: var(--text-color);
    cursor: pointer;
    font-size: 1rem;
    outline: none;
    transition: var(--transition);
}

.btn-primary,
.btn-secondary,
.theme-toggle {
    padding: 10px 20px;
    border: none;
    border-radius: 12px;
    cursor: pointer;
    font-family: inherit;
    font-weight: 500;
    font-size: 0.95rem;
    transition: var(--transition);
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 8px;
    backdrop-filter: blur(4px);
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 8px;
    backdrop-filter: blur(4px);
    gap: 8px;
    backdrop-filter: blur(4px);
    box-shadow: var(--shadow-sm);
    white-space: nowrap;
}

.btn-secondary svg,
.action-btn svg {
    pointer-events: none;
    transition: transform 0.2s;
}



.theme-toggle {
    background: var(--secondary-color);
    color: var(--text-color);
    padding: 10px;
    font-size: 1.2rem;
    border-radius: 50%;
    width: 44px;
    height: 44px;
    /* Reset absolute positioning if it was there */
    position: static;
    margin: 0;
}

.btn-primary {
    background-color: var(--primary-color);
    color: white;
    box-shadow: 0 4px 15px rgba(108, 92, 231, 0.3);
}

.btn-primary:active {
    transform: translateY(0);
}

.btn-secondary {
    background-color: var(--secondary-color);
    color: var(--text-color);
    border: 1px solid var(--glass-border);
}

@media (hover: hover) {
    select:hover {
        background: var(--secondary-hover);
    }

    .theme-toggle:hover,
    .icon-toggle:hover {
        transform: rotate(15deg) scale(1.05);
        background: var(--secondary-hover);
    }

    .btn-primary:hover {
        background-color: var(--primary-hover);
        transform: translateY(-2px);
        box-shadow: 0 6px 20px rgba(108, 92, 231, 0.4);
    }

    .btn-secondary:hover {
        background-color: var(--secondary-hover);
        transform: translateY(-2px);
    }

    .num-btn:hover,
    .action-btn:hover {
        background: var(--primary-color);
        color: white;
        transform: translateY(-4px);
        box-shadow: 0 8px 15px rgba(108, 92, 231, 0.2);
    }

    .btn-secondary:hover svg,
    .action-btn:hover svg {
        transform: scale(1.1);
    }
}

.btn-secondary.active {
    background-color: var(--primary-color);
    color: white;
    border-color: transparent;
}

.game-area {
    width: 100%;
    display: flex;
    flex-direction: column;
    align-items: center;
}

.sudoku-board {
    display: grid;
    grid-template-columns: repeat(9, 1fr);
    grid-template-rows: repeat(9, 1fr);
    width: 100%;
    aspect-ratio: 1;
    background: transparent;
    border: 3px solid var(--board-border);
    gap: 0;
    margin-bottom: 25px;
    border-radius: 16px;
    overflow: hidden;
    box-shadow: var(--shadow-md);
    position: relative;
}

.cell {
    background: var(--glass-bg);
    display: flex;
    justify-content: center;
    align-items: center;
    font-size: 1.8rem;
    font-weight: 500;
    cursor: pointer;
    transition: background-color 0.15s, transform 0.1s;
    user-select: none;
    position: relative;
    border-right: 1px solid var(--cell-border);
    border-bottom: 1px solid var(--cell-border);
    color: var(--text-color);
}

/* Thicker borders for 3x3 boxes using CSS logic */
.cell:nth-child(3n) {
    border-right: 2px solid var(--board-border);
}

.cell:nth-child(9n) {
    border-right: none;
}

.cell:nth-child(n+19):nth-child(-n+27),
.cell:nth-child(n+46):nth-child(-n+54) {
    border-bottom: 2px solid var(--board-border);
}

.cell:nth-child(n+73):nth-child(-n+81) {
    border-bottom: none;
}

.cell.selected {
    background-color: var(--selected-bg) !important;
    color: var(--selected-text) !important;
    font-weight: 600;
}

.cell.selected .notes-grid .note-num {
    color: var(--selected-text) !important;
}

.cell.highlighted {
    background-color: var(--highlight-bg);
}

.cell.fixed {
    color: var(--fixed-num-color);
    font-weight: 600;
}

.cell.user-input {
    color: var(--user-num-color);
    font-weight: 500;
}

.cell.error {
    background-color: #ff7675;
    color: #fff;
    animation: shake 0.4s;
}

/* Notes Grid */
.cell .notes-grid {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    grid-template-rows: repeat(3, 1fr);
    width: 100%;
    height: 100%;
    pointer-events: none;
    padding: 2px;
}

.cell .note-num {
    display: flex;
    justify-content: center;
    align-items: center;
    font-size: 0.6rem;
    color: var(--text-color);
    opacity: 0.7;
    font-weight: 600;
}

.numpad {
    display: grid;
    grid-template-columns: repeat(9, 1fr);
    gap: 6px;
    width: 100%;
    margin-bottom: 20px;
}

.num-btn,
.action-btn {
    padding: 10px 0;
    border: 1px solid var(--glass-border);
    background: var(--glass-bg);
    border-radius: 8px;
    font-size: 1.8rem;
    cursor: pointer;
    transition: var(--transition);
    color: var(--primary-color);
    font-weight: 700;
    box-shadow: var(--shadow-sm);
    backdrop-filter: blur(8px);
}

.num-btn:active,
.action-btn:active {
    transform: translateY(-1px);
}

.num-btn:disabled {
    opacity: 0.5;
    cursor: not-allowed;
    transform: none;
    box-shadow: none;
    background: var(--glass-bg);
    /* Reset to default if needed */
    color: var(--text-color);
}



.timer {
    font-size: 1.5rem;
    font-variant-numeric: tabular-nums;
    color: var(--text-color);
    margin-top: 10px;
    font-weight: 200;
    letter-spacing: 1px;
}

/* Modal */
.modal-overlay {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: rgba(0, 0, 0, 0.6);
    backdrop-filter: blur(5px);
    display: flex;
    justify-content: center;
    align-items: center;
    z-index: 100;
    opacity: 0;
    pointer-events: none;
    transition: opacity 0.3s;
}

.modal-overlay.visible {
    opacity: 1;
    pointer-events: all;
}

.modal {
    background: var(--glass-bg);
    padding: 40px;
    border-radius: 24px;
    text-align: center;
    box-shadow: var(--shadow-md);
    transform: translateY(30px);
    transition: transform 0.4s cubic-bezier(0.175, 0.885, 0.32, 1.275);
    border: 1px solid var(--glass-border);
    backdrop-filter: blur(16px);
    color: var(--text-color);
    max-width: 90%;
}

.modal h2 {
    margin-top: 0;
    font-size: 2rem;
    color: var(--primary-color);
}

.modal-overlay.visible .modal {
    transform: translateY(0);
}

@keyframes shake {
    0% {
        transform: translateX(0);
    }

    25% {
        transform: translateX(-6px);
    }

    50% {
        transform: translateX(6px);
    }

    75% {
        transform: translateX(-6px);
    }

    100% {
        transform: translateX(0);
    }
}

@media (max-width: 500px) {
    .container {
        padding: 4px;
    }

    h1 {
        font-size: 1.8rem;
    }

    .header-top-row {
        padding: 0 5px;
        margin-bottom: 12px;
    }

    .icon-toggle {
        width: 36px;
        height: 36px;
        font-size: 1rem;
    }

    .theme-toggle {
        width: 36px;
        height: 36px;
        font-size: 1rem;
        padding: 8px;
    }

    .header-controls {
        padding: 10px;
        gap: 10px;
        justify-content: center;
        flex-wrap: wrap;
    }

    .control-group {
        gap: 6px;
    }

    /* First row: stats full width centered */
    .stats-group {
        gap: 12px;
        padding: 8px 16px;
        order: -1;
        flex: 0 0 100%;
        justify-content: center;
    }

    /* Second row: other controls evenly distributed */
    .control-group:first-child {
        order: 0;
    }

    .control-group:last-child {
        order: 1;
    }

    .stat-item {
        font-size: 1rem;
        gap: 5px;
    }

    .stat-item svg {
        width: 18px;
        height: 18px;
    }

    .btn-icon-primary,
    .btn-icon-secondary {
        width: 38px;
        height: 38px;
    }

    .btn-icon-primary svg,
    .btn-icon-secondary svg {
        width: 18px;
        height: 18px;
    }

    #difficulty-select {
        padding: 8px 10px;
        font-size: 0.9rem;
    }

    .cell {
        font-size: 1.2rem;
    }

    .numpad {
        gap: 6px;
    }

    .num-btn {
        padding: 12px 0;
        font-size: 1.6rem;
    }

    .controls-middle {
        gap: 10px;
        margin-bottom: 20px;
    }

    .controls-middle button {
        width: 50px;
        font-size: 1.3rem;
    }
}