:root {
    --primary-blue: #1a73e8;
    --primary-dark: #1557b0;
    --bg-gray: #f8f9fa;
    --text-main: #202124;
    --text-secondary: #5f6368;
    --success-green: #28a745;
    --error-red: #dc3545;
    --border-color: #dadce0;
    --white: #ffffff;
    --shadow: 0 2px 4px rgba(0,0,0,0.1);
}

* {
    box-sizing: border-box;
    -webkit-tap-highlight-color: transparent;
}

body {
    margin: 0;
    padding: 0;
    font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
    background-color: var(--bg-gray);
    color: var(--text-main);
    line-height: 1.5;
}

/* Layout Container */
.app-container {
    width: 100%;
    max-width: 600px;
    margin: 0 auto;
    min-height: 100vh;
    background-color: var(--white);
    display: flex;
    flex-direction: column;
    box-shadow: var(--shadow);
}

/* Progress Header */
.header {
    padding: 16px;
    border-bottom: 1px solid var(--border-color);
    position: sticky;
    top: 0;
    background: var(--white);
    z-index: 10;
}

.progress-container {
    width: 100%;
    height: 8px;
    background-color: #e8eaed;
    border-radius: 4px;
    overflow: hidden;
}

.progress-bar {
    height: 100%;
    background-color: var(--primary-blue);
    transition: width 0.3s ease;
}

/* Content Area */
.content {
    padding: 20px;
    flex-grow: 1;
}

.question-text {
    font-size: 1.2rem;
    font-weight: 600;
    margin-bottom: 24px;
    color: var(--text-main);
}

/* Options */
.options-container {
    display: flex;
    flex-direction: column;
    gap: 12px;
}

.option {
    display: flex;
    align-items: center;
    width: 100%;
    min-height: 56px; /* Accessible touch target */
    padding: 12px 16px;
    background-color: var(--white);
    border: 2px solid var(--border-color);
    border-radius: 12px;
    font-size: 1rem;
    text-align: left;
    cursor: pointer;
    transition: all 0.2s ease;
    color: var(--text-main);
}

.option:hover:not(:disabled) {
    border-color: var(--primary-blue);
    background-color: #f1f3f4;
}

.option:active:not(:disabled) {
    transform: scale(0.98);
}

/* Feedback States */
.option.correct {
    background-color: #e6f4ea;
    border-color: var(--success-green);
    color: #137333;
    font-weight: 600;
}

.option.incorrect {
    background-color: #fce8e6;
    border-color: var(--error-red);
    color: #c5221f;
}

.option:disabled {
    cursor: default;
}

/* Explanation Box */
.explanation-box {
    margin-top: 24px;
    padding: 16px;
    border-radius: 8px;
    background-color: #f1f3f4;
    border-left: 4px solid var(--primary-blue);
}

.explanation-title {
    font-weight: 700;
    margin-bottom: 8px;
    display: block;
}

/* Action Buttons */
.actions {
    padding: 20px;
    border-top: 1px solid var(--border-color);
    background: var(--white);
}

.btn-primary {
    width: 100%;
    height: 48px;
    background-color: var(--primary-blue);
    color: white;
    border: none;
    border-radius: 24px;
    font-size: 1rem;
    font-weight: 600;
    cursor: pointer;
    transition: background-color 0.2s;
}

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

.btn-primary:disabled {
    background-color: var(--border-color);
    cursor: not-allowed;
}

/* Desktop Adjustments */
@media (min-width: 768px) {
    body {
        padding: 40px 20px;
    }
    
    .app-container {
        border-radius: 16px;
        min-height: auto;
    }
}