:root {
    --bg-color: #1a1a1a;
    --primary-color: #646cff;
    --accent-color: #ff4081;
    --text-color: #ffffff;
    --panel-bg: rgba(255, 255, 255, 0.1);
    --font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
}

html, body {
    margin: 0;
    padding: 0;
    width: 100%;
    height: 100%;
    overflow: hidden;
}

body {
    font-family: var(--font-family);
    color: var(--text-color);
    background: radial-gradient(circle at center, #2a2a2a 0%, #1a1a1a 100%);
    user-select: none;
    -webkit-user-select: none;
}

#ui-overlay {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    pointer-events: none;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
}

.screen {
    pointer-events: auto;
    background: rgba(0, 0, 0, 0.85);
    padding: 2rem;
    border-radius: 16px;
    text-align: center;
    backdrop-filter: blur(10px);
    box-shadow: 0 8px 32px rgba(0, 0, 0, 0.3);
    animation: fadeIn 0.3s ease-out;
}

.hidden {
    display: none !important;
}

h1 {
    font-size: 3rem;
    margin-bottom: 2rem;
    background: linear-gradient(45deg, var(--primary-color), var(--accent-color));
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    text-shadow: 0 4px 8px rgba(0,0,0,0.2);
}

.btn {
    background: linear-gradient(135deg, var(--primary-color), #535bf2);
    border: none;
    padding: 12px 24px;
    margin: 8px;
    border-radius: 8px;
    color: white;
    font-size: 1.1rem;
    cursor: pointer;
    transition: transform 0.1s, box-shadow 0.2s;
    font-weight: bold;
    text-transform: uppercase;
    letter-spacing: 1px;
}

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

.btn:hover {
    box-shadow: 0 0 15px rgba(100, 108, 255, 0.5);
}

#game-ui {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    pointer-events: none;
    display: flex;
    flex-direction: column;
    justify-content: space-between;
}

.top-bar {
    pointer-events: auto;
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 10px 20px;
    background: linear-gradient(to bottom, rgba(0,0,0,0.8), transparent);
}

#timer {
    font-size: 1.5rem;
    font-variant-numeric: tabular-nums;
    font-weight: bold;
    text-shadow: 0 2px 4px rgba(0,0,0,0.5);
}

.icon-btn {
    background: none;
    border: none;
    color: white;
    font-size: 1.5rem;
    cursor: pointer;
    opacity: 0.8;
    transition: opacity 0.2s;
}

.icon-btn:hover {
    opacity: 1;
}

.numpad {
    pointer-events: auto;
    display: grid;
    grid-template-columns: repeat(5, 1fr); /* 1-5, 6-0 */
    gap: 8px;
    padding: 10px;
    background: rgba(0, 0, 0, 0.8);
    border-radius: 12px 12px 0 0;
    width: 100%;
    max-width: 500px;
    margin: 0 auto;
    box-sizing: border-box;
    transition: transform 0.3s cubic-bezier(0.175, 0.885, 0.32, 1.275);
}

.numpad button {
    background: rgba(255, 255, 255, 0.1);
    border: 1px solid rgba(255, 255, 255, 0.2);
    color: white;
    font-size: 1.5rem;
    padding: 15px 0;
    border-radius: 8px;
    cursor: pointer;
}

.numpad button:active {
    background: rgba(255, 255, 255, 0.3);
}

.numpad button.clear-btn {
    color: #ff4081;
    border-color: #ff4081;
}

@keyframes fadeIn {
    from { opacity: 0; transform: translateY(20px); }
    to { opacity: 1; transform: translateY(0); }
}

@media (min-width: 600px) {
    .numpad {
        width: 300px;
        grid-template-columns: repeat(3, 1fr);
        position: absolute;
        right: 20px;
        bottom: 20px;
        border-radius: 12px;
        padding: 15px;
    }

    .numpad button:last-child {
        grid-column: span 3; /* Clear button spans full width on desktop layout */
    }
}