:root {
    --primary-color: #6D28D9; /* Violet 700 */
    --primary-color-light: #8B5CF6; /* Violet 500 */
    --secondary-color: #EDE9FE; /* Violet 100 */
    --text-color: #1F2937; /* Gray 800 */
    --text-light-color: #4B5563; /* Gray 600 */
    --bg-color: #f8f7fc; /* Lighter grayish background */
    --card-bg-color: #FFFFFF;
    --success-color: #10B981; /* Emerald 500 */
    --error-color: #EF4444; /* Red 500 */
    --border-color: #E5E7EB; /* Gray 200 */
    --font-family: 'Noto Sans SC', 'Helvetica Neue', Arial, sans-serif;
    --border-radius: 12px;
    --shadow: 0 10px 25px -5px rgba(0, 0, 0, 0.07), 0 20px 20px -5px rgba(0, 0, 0, 0.04);
}

body {
    font-family: var(--font-family);
    background-color: var(--bg-color);
    color: var(--text-color);
    margin: 0;
    padding: 20px;
    display: flex;
    justify-content: center;
    align-items: flex-start;
    min-height: 100vh;
    box-sizing: border-box;
    -webkit-font-smoothing: antialiased;
    -moz-osx-font-smoothing: grayscale;
}

.app-container {
    width: 100%;
    max-width: 600px;
}

.app-header h1 {
    font-size: 2.25rem; /* 36px */
    font-weight: 700;
    color: var(--primary-color);
    text-align: center;
    margin-bottom: 2rem; /* 32px */
}

.operation-selector {
    display: flex;
    flex-wrap: wrap;
    justify-content: center;
    gap: 0.75rem; /* 12px */
    margin-bottom: 2rem;
}

.op-btn {
    background-color: var(--card-bg-color);
    color: var(--primary-color-light);
    border: 1px solid var(--secondary-color);
    padding: 0.6rem 1.2rem;
    border-radius: var(--border-radius);
    font-size: 0.9rem;
    font-weight: 500;
    cursor: pointer;
    transition: all 0.2s ease-in-out;
    box-shadow: 0 1px 3px 0 rgba(0,0,0,0.03), 0 1px 2px -1px rgba(0,0,0,0.03);
}

.op-btn:hover {
    background-color: var(--secondary-color);
    border-color: var(--primary-color-light);
}

.op-btn.active {
    background-color: var(--primary-color);
    color: var(--card-bg-color);
    border-color: var(--primary-color);
    box-shadow: 0 4px 6px -1px rgba(0,0,0,0.05), 0 2px 4px -2px rgba(0,0,0,0.05);
}

.quiz-card {
    background-color: var(--card-bg-color);
    border-radius: var(--border-radius);
    padding: 1.75rem; /* 28px */
    box-shadow: var(--shadow);
    margin-bottom: 1.5rem;
}

.quiz-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 1.5rem;
    color: var(--text-light-color);
    font-size: 0.9rem;
}

#score-display {
    display: flex;
    align-items: center;
    gap: 0.3rem;
    font-weight: 500;
}

#score-display .icon {
    color: #FBBF24; /* Amber 400 for trophy icon */
}

.problem-container {
    min-height: 80px; /* Ensure consistent height */
    display: flex;
    align-items: center;
    justify-content: center;
    margin-bottom: 1.5rem;
}

#problem-statement {
    font-size: 2.5rem; /* 40px */
    font-weight: 700;
    text-align: center;
    color: var(--text-color);
    line-height: 1.3;
}

#answer-input {
    width: 100%;
    padding: 0.875rem 1rem; /* 14px 16px */
    border: 1px solid var(--border-color);
    border-radius: 0.75rem; /* 8px */
    font-size: 1.25rem; /* 20px */
    text-align: center;
    box-sizing: border-box;
    transition: border-color 0.2s ease, box-shadow 0.2s ease;
}

#answer-input:focus {
    outline: none;
    border-color: var(--primary-color-light);
    box-shadow: 0 0 0 3px var(--secondary-color);
}

.feedback-area {
    min-height: 24px;
    margin-top: 1rem;
    text-align: center;
    font-weight: 500;
    font-size: 1rem;
}

.feedback-area.correct {
    color: var(--success-color);
}

.feedback-area.incorrect {
    color: var(--error-color);
}

.action-buttons {
    display: flex;
    gap: 1rem; /* 16px */
    justify-content: center;
}

.btn {
    padding: 0.875rem 1.5rem; /* 14px 24px */
    border-radius: var(--border-radius);
    font-size: 1rem;
    font-weight: 500;
    cursor: pointer;
    transition: all 0.2s ease-in-out;
    border: none;
    display: flex;
    align-items: center;
    gap: 0.5rem; /* 8px */
    box-shadow: 0 1px 3px 0 rgba(0,0,0,0.04), 0 1px 2px -1px rgba(0,0,0,0.04);
}
.btn .icon {
    stroke-width: 2.5px;
}

.btn-primary {
    background-color: var(--primary-color);
    color: white;
}

.btn-primary:hover {
    background-color: #5B21B6; /* Violet 800 */
    box-shadow: 0 4px 6px -1px rgba(0,0,0,0.05), 0 2px 4px -2px rgba(0,0,0,0.05);
}

.btn-primary:disabled {
    background-color: #A78BFA; /* Violet 300 */
    cursor: not-allowed;
    opacity: 0.7;
}


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

.btn-secondary:hover {
    background-color: #DDD6FE; /* Violet 200 */
    border-color: var(--primary-color);
}

/* Responsive adjustments */
@media (max-width: 480px) {
    body {
        padding: 15px;
    }
    .app-header h1 {
        font-size: 1.8rem; /* Smaller title */
    }
    .op-btn {
        padding: 0.5rem 0.8rem;
        font-size: 0.8rem;
    }
    #problem-statement {
        font-size: 2rem; /* Smaller problem text */
    }
    #answer-input {
        font-size: 1.1rem;
    }
    .btn {
        padding: 0.75rem 1rem;
        font-size: 0.9rem;
    }
    .action-buttons {
        flex-direction: column; /* Stack buttons on small screens */
    }
}