:root {
    --bg-color-light: #f5f5f5;
    --text-color-light: #333333;
    --card-bg-color-light: #ffffff;
    --border-color-light: #dddddd;
    --primary-color-light: #007bff;
    --primary-hover-color-light: #0056b3;
    --correct-color-light: #28a745;
    --incorrect-color-light: #dc3545;

    --bg-color-dark: #121212;
    --text-color-dark: #e0e0e0;
    --card-bg-color-dark: #1e1e1e;
    --border-color-dark: #444444;
    --primary-color-dark: #1e90ff;
    --primary-hover-color-dark: #0073e6;
    --correct-color-dark: #32cd32;
    --incorrect-color-dark: #ff4500;
}

body {
    font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell, 'Open Sans', 'Helvetica Neue', sans-serif;
    margin: 0;
    transition: background-color 0.3s, color 0.3s;
}

.dark-mode {
    background-color: var(--bg-color-dark);
    color: var(--text-color-dark);
}

.light-mode {
    background-color: var(--bg-color-light);
    color: var(--text-color-light);
}

header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 1rem 2rem;
    border-bottom: 1px solid;
}

.dark-mode header { border-color: var(--border-color-dark); }
.light-mode header { border-color: var(--border-color-light); }

header h1 {
    font-size: 1.5rem;
    margin: 0;
}

.header-btn {
    background: none;
    border: none;
    font-size: 1.5rem;
    cursor: pointer;
    padding: 0.5rem;
}
.dark-mode .header-btn { color: var(--text-color-dark); }
.light-mode .header-btn { color: var(--text-color-light); }

main {
    padding: 2rem;
    text-align: center;
}

.screen {
    display: none;
}

.screen.active {
    display: block;
}

#home-screen h2 {
    font-size: 2rem;
    margin-bottom: 2rem;
}

.difficulty-options-grid {
    display: grid;
    grid-template-columns: repeat(2, 1fr);
    gap: 1rem;
    max-width: 600px;
    margin: 0 auto 1rem auto;
}

.difficulty-options-advanced {
    display: flex;
    justify-content: center;
    margin-bottom: 2rem;
}

.difficulty-options-advanced .difficulty-btn {
    width: 100%;
    max-width: 600px;
}

.difficulty-btn {
    padding: 1rem 2rem;
    font-size: 1.2rem;
    cursor: pointer;
    border: 1px solid;
    border-radius: 8px;
    transition: background-color 0.3s, transform 0.2s;
}

.dark-mode .difficulty-btn {
    background-color: var(--card-bg-color-dark);
    color: var(--text-color-dark);
    border-color: var(--primary-color-dark);
}

.light-mode .difficulty-btn {
    background-color: var(--card-bg-color-light);
    color: var(--primary-color-light);
    border-color: var(--primary-color-light);
}

.difficulty-btn:hover {
    transform: translateY(-3px);
}

.dark-mode .difficulty-btn:hover { background-color: var(--primary-color-dark); color: var(--card-bg-color-dark); }
.light-mode .difficulty-btn:hover { background-color: var(--primary-color-light); color: var(--card-bg-color-light); }


.info {
    margin-top: 3rem;
    padding: 1.5rem;
    border-radius: 8px;
    display: inline-block;
}

.dark-mode .info { background-color: var(--card-bg-color-dark); }
.light-mode .info { background-color: var(--card-bg-color-light); }


.info p {
    margin: 0.5rem 0;
    text-align: left;
}

#game-screen {
    max-width: 600px;
    margin: 0 auto;
}

#game-info {
    display: flex;
    justify-content: space-between;
    align-items: center;
    font-size: 1.2rem;
    margin-bottom: 1rem;
    padding: 0 1rem;
}

#timer-display {
    font-weight: bold;
    font-size: 1.5rem;
}

#staff-container {
    position: relative;
    height: 150px;
    margin-bottom: 2rem;
    display: flex;
    align-items: center;
    justify-content: center;
}

.staff-line {
    position: absolute;
    width: 100%;
    height: 2px;
    background-color: var(--text-color);
}

.dark-mode .staff-line { background-color: var(--text-color-dark); }
.light-mode .staff-line { background-color: var(--text-color-light); }

.clef {
    position: absolute;
    left: 10px;
    font-size: 80px;
    line-height: 1;
}

.note {
    position: absolute;
    left: 45%;
    width: 20px;
    height: 20px;
    background-color: var(--text-color);
    border-radius: 50%;
}
.dark-mode .note { background-color: var(--text-color-dark); }
.light-mode .note { background-color: var(--text-color-light); }


.ledger-line {
    position: absolute;
    left: 42%;
    width: 30px;
    height: 2px;
    background-color: var(--text-color);
}
.dark-mode .ledger-line { background-color: var(--text-color-dark); }
.light-mode .ledger-line { background-color: var(--text-color-light); }

#answer-options {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(80px, 1fr));
    gap: 1rem;
}

.answer-btn {
    padding: 1rem;
    font-size: 1.5rem;
    cursor: pointer;
    border: 1px solid;
    border-radius: 8px;
    transition: background-color 0.3s;
}

.dark-mode .answer-btn {
    background-color: var(--card-bg-color-dark);
    color: var(--text-color-dark);
    border-color: var(--border-color-dark);
}

.light-mode .answer-btn {
    background-color: var(--card-bg-color-light);
    color: var(--text-color-light);
    border-color: var(--border-color-light);
}

.dark-mode .answer-btn:hover { background-color: #333; }
.light-mode .answer-btn:hover { background-color: #eee; }

#feedback {
    margin-top: 1rem;
    font-size: 1.2rem;
    font-weight: bold;
    height: 2rem;
}

.correct { color: var(--correct-color-light); }
.incorrect { color: var(--incorrect-color-light); }

.dark-mode .correct { color: var(--correct-color-dark); }
.dark-mode .incorrect { color: var(--incorrect-color-dark); }

/* --- Responsive Design --- */
@media (max-width: 600px) {
    header {
        padding: 0.5rem 1rem;
    }
    header h1 {
        font-size: 1.2rem;
    }
    main {
        padding: 1rem;
    }
    #home-screen h2 {
        font-size: 1.5rem;
    }
    .difficulty-options-grid {
        grid-template-columns: 1fr;
    }
    .difficulty-btn {
        font-size: 1rem;
        padding: 0.8rem 1rem;
    }
    .info {
        margin-top: 2rem;
        padding: 1rem;
    }
    .info p {
        font-size: 0.9rem;
    }
    .answer-btn {
        font-size: 1.2rem;
    }
    .clef {
        font-size: 70px;
        left: 0;
    }
}