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

body {
    font-family: 'Roboto', sans-serif;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    background: linear-gradient(135deg, #e0e7ff 50%, #d9e6ff 50%);
    position: relative;
    overflow: hidden;
    transition: background 0.3s;
}

body::before, body::after {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    opacity: 0.15;
    z-index: -1;
}

body::before {
    background: radial-gradient(circle at 30% 30%, #a0d6ff 0%, transparent 70%);
    animation: waveMove1 15s linear infinite;
}

body::after {
    background: radial-gradient(circle at 70% 70%, #ffcccb 0%, transparent 70%);
    animation: waveMove2 20s linear infinite;
}

@keyframes waveMove1 {
    0% { transform: translate(0, 0); }
    50% { transform: translate(-50px, 50px); }
    100% { transform: translate(0, 0); }
}

@keyframes waveMove2 {
    0% { transform: translate(0, 0); }
    50% { transform: translate(50px, -50px); }
    100% { transform: translate(0, 0); }
}

body.dark {
    background: linear-gradient(135deg, #1e2a44 50%, #34495e 50%);
}

body.dark::before {
    background: radial-gradient(circle at 30% 30%, #4a90e2 0%, transparent 70%);
}

body.dark::after {
    background: radial-gradient(circle at 70% 70%, #e74c3c 0%, transparent 70%);
}

.container {
    text-align: center;
    background: rgba(255, 255, 255, 0.95);
    padding: 25px;
    border-radius: 15px;
    box-shadow: 0 4px 20px rgba(0, 0, 0, 0.15);
    transition: background 0.3s;
    margin: 20px;
    z-index: 1;
    position: relative;
    max-width: 450px;
    width: 100%;
    display: flex;
    flex-direction: column;
    align-items: center;
}

body.dark .container {
    background: rgba(52, 73, 94, 0.95);
}

h1 {
    font-family: 'Orbitron', sans-serif;
    font-size: 2.5em;
    margin-bottom: 20px;
    color: #2c3e50;
    text-shadow: 0 2px 4px rgba(0, 0, 0, 0.1);
}

body.dark h1 {
    color: #ecf0f1;
    text-shadow: 0 2px 4px rgba(0, 0, 0, 0.3);
}

.color-picker {
    display: flex;
    gap: 20px;
    justify-content: center;
    margin-bottom: 20px;
}

.color-picker label {
    display: flex;
    align-items: center;
    gap: 8px;
    font-size: 0.9em;
    color: #34495e;
}

body.dark .color-picker label {
    color: #bdc3c7;
}

#status {
    font-size: 1.5em;
    font-weight: 500;
    margin-bottom: 20px;
    color: #34495e;
    transition: all 0.3s;
    position: relative;
    z-index: 2;
}

body.dark #status {
    color: #ecf0f1;
}

#status.win {
    font-size: 2em;
    font-weight: bold;
    animation: tada 1s ease;
}

@keyframes tada {
    0% { transform: scale(1); }
    10%, 20% { transform: scale(0.9) rotate(-3deg); }
    30%, 50%, 70%, 90% { transform: scale(1.2) rotate(3deg); }
    40%, 60%, 80% { transform: scale(1.2) rotate(-3deg); }
    100% { transform: scale(1) rotate(0); }
}

.particles {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    pointer-events: none;
    z-index: 1;
}

.particle {
    position: absolute;
    width: 8px;
    height: 8px;
    background: #e74c3c;
    border-radius: 50%;
    opacity: 0.8;
    animation: sparkle 1s ease-out forwards;
}

@keyframes sparkle {
    0% {
        transform: scale(1) translate(0, 0);
        opacity: 0.8;
    }
    100% {
        transform: scale(0) translate(var(--x), var(--y));
        opacity: 0;
    }
}

#board {
    --board-size: min(80vw, 400px); /* Responsive board size */
    width: var(--board-size);
    height: var(--board-size);
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    grid-template-rows: repeat(3, 1fr);
    grid-gap: 6px;
    background-color: #dfe4ea;
    padding: 6px;
    border-radius: 12px;
    position: relative;
    margin: 0 auto;
    box-shadow: 0 4px 20px rgba(0, 0, 0, 0.15);
    transition: background-color 0.3s, box-shadow 0.3s;
    z-index: 2;
    border: 3px solid #b2bec3;
}

body.dark #board {
    background-color: #636e72;
    border-color: #2d3436;
}

.cell {
    width: 100%;
    height: 100%;
    background-color: #fff;
    border: 3px solid #b2bec3;
    display: flex;
    justify-content: center;
    align-items: center;
    font-size: calc(var(--board-size) / 6); /* Scale font with board */
    font-family: 'Orbitron', sans-serif;
    cursor: pointer;
    transition: background-color 0.3s, box-shadow 0.3s;
    box-shadow: 0 0 8px rgba(0, 255, 255, 0.3);
    border-radius: 8px;
    position: relative;
    z-index: 2;
}

body.dark .cell {
    background-color: #dfe6e9;
    border-color: #2d3436;
    box-shadow: 0 0 8px rgba(26, 163, 255, 0.459);
}

.cell:hover:not(.x):not(.o) {
    background-color: #f1f2f6;
}

body.dark .cell:hover:not(.x):not(.o) {
    background-color: #b2bec3;
}

.cell.x, .cell.o {
    animation: symbolPop 0.5s ease-in;
}

@keyframes symbolPop {
    0% { transform: scale(0) rotate(0deg); opacity: 0; }
    50% { transform: scale(1.2) rotate(10deg); }
    100% { transform: scale(1) rotate(0deg); opacity: 1; }
}

#win-line {
    position: absolute;
    background-color: transparent;
    transform-origin: 0 0;
    transition: width 0.5s ease-in-out;
    z-index: 3;
    height: 10px; /* Fixed thickness */
}

#win-line.x {
    background-color: #e74c3c;
}

#win-line.o {
    background-color: #3498db;
}

.buttons {
    display: flex;
    gap: 15px;
    justify-content: center;
    margin-top: 20px;
}

#reset, #theme-toggle {
    padding: 12px 24px;
    font-size: 1em;
    font-weight: 500;
    border: none;
    border-radius: 8px;
    cursor: pointer;
    transition: background-color 0.3s, transform 0.2s;
}

#reset {
    background-color: #2ecc71;
    color: white;
}

#reset:hover {
    background-color: #27ae60;
    transform: translateY(-2px);
}

#theme-toggle {
    background-color: #3498db;
    color: white;
}

#theme-toggle:hover {
    background-color: #2980b9;
    transform: translateY(-2px);
}

footer {
    margin-top: 20px;
    font-size: 1em;
    color: #2c3e50;
    text-align: center;
    z-index: 1;
    font-family: 'Roboto', sans-serif;
}

body.dark footer {
    color: #ecf0f1;
}

@media (max-width: 600px) {
    #board {
        --board-size: min(90vw, 320px);
    }
    .cell {
        font-size: calc(var(--board-size) / 6);
        border-width: 2px;
        box-shadow: 0 0 6px rgba(0, 255, 255, 0.3);
    }
    body.dark .cell {
        box-shadow: 0 0 6px rgba(26, 163, 255, 0.459);
    }
    .container {
        margin: 15px;
        padding: 20px;
    }
    #status.win {
        font-size: 1.8em;
    }
    .particle {
        width: 6px;
        height: 6px;
    }
    h1 {
        font-size: 2em;
    }
}

@media (max-width: 400px) {
    #board {
        --board-size: min(90vw, 240px);
    }
    .cell {
        font-size: calc(var(--board-size) / 6);
        border-width: 2px;
        box-shadow: 0 0 4px rgba(0, 255, 255, 0.3);
    }
    body.dark .cell {
        box-shadow: 0 0 4px rgba(26, 163, 255, 0.459);
    }
    .container {
        margin: 10px;
        padding: 15px;
    }
    .color-picker {
        flex-direction: column;
        gap: 12px;
    }
    #status {
        font-size: 1.2em;
    }
    #status.win {
        font-size: 1.5em;
    }
    .particle {
        width: 4px;
        height: 4px;
    }
    h1 {
        font-size: 1.8em;
    }
    .buttons {
        flex-direction: column;
        gap: 10px;
    }
    #reset, #theme-toggle {
        padding: 10px 20px;
    }
}