:root {
    /* Dark theme (default) */
    --primary-color: #6a11cb;
    --secondary-color: #2575fc;
    --background-color: #0f0f23;
    --container-bg: rgba(255, 255, 255, 0.08); /* Darker glassmorphism */
    --container-blur: blur(20px); /* Slightly more blur for dark */
    --container-border: 1.5px solid rgba(255,255,255,0.15); /* Subtle border */
    --cell-bg: rgba(255, 255, 255, 0.12); /* Dark glassy cell */
    --cell-hover-bg: rgba(255, 255, 255, 0.18); /* Dark glassy hover */
    --text-color: #ffffff;
    --border-color: rgba(255, 255, 255, 0.2);
    --x-color: #ff6b9d;
    --o-color: #00e5ff;
    --winning-cell-bg: rgba(76, 175, 80, 0.3);
    --shadow-color: rgba(0, 0, 0, 0.3);
    --glass-shadow: 0 8px 32px 0 rgba(0, 0, 0, 0.4);
    --glass-inner-shadow: 0 1.5px 8px 0 rgba(0, 0, 0, 0.2) inset;
    --glass-bg-overlay: rgba(255,255,255,0.05);
    --glass-bg-gradient: linear-gradient(120deg, rgba(106,17,203,0.15) 0%, rgba(37,117,252,0.15) 100%);
    --font-family: 'Poppins', sans-serif;
}

/* Light theme */
:root.light-theme {
    --background-color: #f4f4f9;
    --container-bg: rgba(255, 255, 255, 0.25);
    --container-blur: blur(18px);
    --container-border: 1.5px solid rgba(255,255,255,0.35);
    --cell-bg: rgba(233, 233, 253, 0.45);
    --cell-hover-bg: rgba(208, 208, 245, 0.55);
    --text-color: #333333;
    --border-color: #d1d1e0;
    --x-color: #ff416c;
    --o-color: #1565c0;
    --winning-cell-bg: rgba(170, 255, 170, 0.5);
    --shadow-color: rgba(0, 0, 0, 0.08);
    --glass-shadow: 0 8px 32px 0 rgba(31, 38, 135, 0.18);
    --glass-inner-shadow: 0 1.5px 8px 0 rgba(31, 38, 135, 0.10) inset;
    --glass-bg-overlay: rgba(255,255,255,0.12);
    --glass-bg-gradient: linear-gradient(120deg, rgba(106,17,203,0.10) 0%, rgba(37,117,252,0.10) 100%);
}

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
    font-family: var(--font-family);
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    background-color: var(--background-color);
    background-image: linear-gradient(135deg, var(--primary-color) 0%, var(--secondary-color) 100%);
    color: var(--text-color);
    padding: 20px;
    /* Glassmorphism background overlay */
    /* Optionally add a subtle overlay for extra glass effect */
}

.glass-body {
    position: relative;
    min-height: 100vh;
    overflow: hidden;
}

.glass-bg {
    position: fixed;
    top: 0; left: 0; right: 0; bottom: 0;
    z-index: 0;
    pointer-events: none;
    background: var(--glass-bg-overlay);
    backdrop-filter: blur(24px) saturate(1.2);
    -webkit-backdrop-filter: blur(24px) saturate(1.2);
    background-image: var(--glass-bg-gradient);
}

.game-container {
    z-index: 1;
    position: relative;
    background: var(--container-bg);
    backdrop-filter: var(--container-blur);
    -webkit-backdrop-filter: var(--container-blur);
    border-radius: 18px;
    border: var(--container-border);
    box-shadow: var(--glass-shadow);
    padding: 32px 44px;
    text-align: center;
    max-width: 450px;
    width: 90%;
    transition: all 0.3s ease;
    position: relative;
    overflow: hidden;
}

h1 {
    margin-bottom: 25px;
    font-weight: 600;
    font-size: 2.5em;
    background: linear-gradient(135deg, #ff6b9d, #4fc3f7);
    background-clip: text;
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    text-shadow: 0 0 20px rgba(255, 107, 157, 0.3);
}

/* Light theme gradient text */
:root.light-theme h1 {
    background: linear-gradient(135deg, var(--primary-color), var(--secondary-color));
    background-clip: text;
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    text-shadow: none;
}

.controls {
    margin-bottom: 25px;
}

.mode-selector {
    display: flex;
    flex-direction: column;
    align-items: flex-start;
    gap: 10px;
    margin-bottom: 20px;
    padding: 15px;
    background: rgba(255,255,255,0.18); /* Glassy background */
    border-radius: 10px;
    border: 1.2px solid rgba(255,255,255,0.25);
    box-shadow: var(--glass-inner-shadow);
    backdrop-filter: blur(8px);
    -webkit-backdrop-filter: blur(8px);
}

.mode-selector label {
    cursor: pointer;
    display: flex;
    align-items: center;
    gap: 8px;
    font-weight: 400;
    color: var(--text-color);
    transition: color 0.2s ease;
}
.mode-selector label:hover {
    color: var(--text-color);
}

.mode-selector input[type="radio"] {
    accent-color: var(--primary-color); /* Modern browser styling for radio */
     transform: scale(1.1); /* Slightly larger radio */
}

button {
    background: rgba(255,255,255,0.18);
    background-image: linear-gradient(135deg, var(--primary-color) 0%, var(--secondary-color) 100%);
    color: white;
    border: 1.5px solid rgba(255,255,255,0.25);
    padding: 12px 25px;
    border-radius: 25px;
    font-size: 1em;
    font-weight: 600;
    cursor: pointer;
    transition: transform 0.2s ease, box-shadow 0.2s ease, background 0.2s;
    box-shadow: 0 4px 24px rgba(31, 38, 135, 0.10);
    backdrop-filter: blur(6px);
    -webkit-backdrop-filter: blur(6px);
}

button:hover {
    transform: translateY(-2px);
    box-shadow: 0 6px 20px rgba(0, 0, 0, 0.15);
}

button:active {
    transform: translateY(0px);
    box-shadow: 0 4px 15px rgba(0, 0, 0, 0.1);
}

/* Theme Toggle Button */
.theme-toggle {
    position: fixed;
    top: 20px;
    right: 20px;
    z-index: 1000;
    background: var(--container-bg);
    border: var(--container-border);
    border-radius: 50px;
    padding: 12px 16px;
    cursor: pointer;
    backdrop-filter: var(--container-blur);
    -webkit-backdrop-filter: var(--container-blur);
    box-shadow: var(--glass-shadow);
    transition: all 0.3s ease;
    display: flex;
    align-items: center;
    gap: 8px;
    font-size: 0.9em;
    font-weight: 500;
    color: var(--text-color);
    min-width: auto;
    background-image: none;
}

.theme-toggle:hover {
    transform: translateY(-1px);
    box-shadow: 0 6px 25px rgba(0, 0, 0, 0.2);
    background: rgba(255, 255, 255, 0.15);
}

.theme-toggle:active {
    transform: translateY(0px);
}

.theme-icon {
    font-size: 1.2em;
    transition: transform 0.3s ease;
}

.theme-toggle:hover .theme-icon {
    transform: rotate(180deg);
}

.board {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    grid-template-rows: repeat(3, 1fr);
    gap: 12px;
    width: 300px;
    height: 300px;
    aspect-ratio: 1 / 1; /* Always square board */
    margin: 25px auto;
    background: rgba(255,255,255,0.18);
    padding: 14px;
    border-radius: 14px;
    box-shadow: var(--glass-inner-shadow);
    border: 1.2px solid rgba(255,255,255,0.22);
    backdrop-filter: blur(8px);
    -webkit-backdrop-filter: blur(8px);
}

.cell {
    background: var(--cell-bg);
    border-radius: 10px;
    display: flex;
    justify-content: center;
    align-items: center;
    font-size: 3.5em;
    font-weight: 600;
    cursor: pointer;
    transition: background 0.3s, box-shadow 0.2s;
    user-select: none;
    box-shadow: 0 2px 8px rgba(31, 38, 135, 0.08);
    border: 1.2px solid rgba(255,255,255,0.18);
    backdrop-filter: blur(4px);
    -webkit-backdrop-filter: blur(4px);
    aspect-ratio: 1 / 1; /* Always square */
    min-width: 0;
    min-height: 0;
    width: 100%;
    height: 100%;
    max-width: 100%;
    max-height: 100%;
    box-sizing: border-box;
}

.cell:not(.occupied):hover {
    background: var(--cell-hover-bg);
    box-shadow: 0 4px 16px rgba(31, 38, 135, 0.12);
}

.cell.x {
    color: var(--x-color);
}

.cell.o {
    color: var(--o-color);
}

.cell.winning-cell {
    background-color: var(--winning-cell-bg);
    animation: pulse 0.8s infinite alternate;
}

@keyframes pulse {
    from { transform: scale(1); }
    to { transform: scale(1.05); }
}

#statusDisplay {
    margin-top: 20px;
    font-size: 1.1em;
    font-weight: 400;
    min-height: 1.5em; /* Prevent layout shifts */
    color: var(--text-color);
}

.player-symbol-selector {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 10px;
    margin-top: 15px;
    margin-bottom: 20px;
    padding: 15px;
    background: rgba(240, 232, 255, 0.35); /* Glassy background */
    border-radius: 10px;
    border: 1.2px solid rgba(255,255,255,0.22);
    box-shadow: var(--glass-inner-shadow);
    backdrop-filter: blur(8px);
    -webkit-backdrop-filter: blur(8px);
}

.player-symbol-selector p {
    margin-bottom: 8px;
    font-weight: 600;
    color: var(--text-color);
}

.player-symbol-selector label {
    cursor: pointer;
    display: flex;
    align-items: center;
    gap: 8px;
    font-weight: 400;
    color: var(--text-color);
    transition: color 0.2s ease;
}
.player-symbol-selector label:hover {
    color: var(--text-color);
}

.player-symbol-selector input[type="radio"] {
    accent-color: var(--primary-color);
    transform: scale(1.1);
}

/* Ensure .hidden class is defined if not already */
.hidden {
    display: none !important; /* Use !important if needed to override other display properties */
}

/* Responsive adjustments */
@media (max-width: 480px) {
    .game-container {
        padding: 14px;
    }
    h1 {
        font-size: 2em;
    }
    .board {
        width: 210px;
        height: 210px;
        aspect-ratio: 1 / 1;
        padding: 8px;
    }
    .cell {
        font-size: 2.2em;
        aspect-ratio: 1 / 1;
        min-width: 0;
        min-height: 0;
        width: 100%;
        height: 100%;
        max-width: 100%;
        max-height: 100%;
        box-sizing: border-box;
    }
    button {
        padding: 8px 16px;
        font-size: 0.9em;
    }
    .mode-selector {
        align-items: center;
    }
}