:root {
    --color-neutral-50: #fafafa;
    --color-neutral-100: #f5f5f5;
    --color-neutral-200: #e5e5e5;
    --color-neutral-300: #d4d4d4;
    --color-neutral-400: #a3a3a3;
    --color-neutral-500: #737373;
    --color-neutral-600: #525252;
    --color-neutral-700: #404040;
    --color-neutral-800: #262626;
    --color-neutral-900: #171717;
    
    --color-primary: #2563eb;
    --color-primary-light: #3b82f6;
    --color-primary-dark: #1d4ed8;
    
    --color-success: #059669;
    --color-warning: #d97706;
    --color-error: #dc2626;
}

*,
*::before,
*::after {
    box-sizing: border-box;
}

* {
    margin: 0;
    padding: 0;
}

html {
    font-size: 16px;
    line-height: 1.6;
}

body {
    font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
    color: var(--color-neutral-900);
    background-color: var(--color-neutral-50);
    min-height: 100vh;
    display: flex;
    flex-direction: column;
}

h1 {
    font-size: 2rem;
    font-weight: 600;
    line-height: 1.2;
    letter-spacing: -0.025em;
}

h2 {
    font-size: 1.5rem;
    font-weight: 600;
    line-height: 1.3;
    letter-spacing: -0.02em;
}

p {
    line-height: 1.6;
}

.header {
    padding: 24px 16px;
    text-align: center;
    border-bottom: 1px solid var(--color-neutral-200);
    background-color: #ffffff;
}

.header__title {
    color: var(--color-primary);
    margin-bottom: 8px;
}

.header__subtitle {
    color: var(--color-neutral-600);
    font-size: 0.875rem;
}

.main {
    flex: 1;
    padding: 24px 16px;
    max-width: 800px;
    margin: 0 auto;
    width: 100%;
}

.footer {
    padding: 16px;
    text-align: center;
    border-top: 1px solid var(--color-neutral-200);
    margin-top: auto;
    background-color: #ffffff;
}

.footer__text {
    color: var(--color-neutral-500);
    font-size: 0.75rem;
}

.screen {
    animation: fadeIn 0.3s ease-out;
}

@keyframes fadeIn {
    from {
        opacity: 0;
        transform: translateY(8px);
    }
    to {
        opacity: 1;
        transform: translateY(0);
    }
}

.screen__title {
    text-align: center;
    margin-bottom: 16px;
    color: var(--color-neutral-900);
}

.screen__description {
    text-align: center;
    color: var(--color-neutral-600);
    margin-bottom: 24px;
    max-width: 500px;
    margin-left: auto;
    margin-right: auto;
}

.screen--start {
    text-align: center;
    max-width: 500px;
    margin: 0 auto;
    padding-top: 48px;
}

.topic-selector {
    margin-bottom: 24px;
}

.topic-selector__label {
    display: block;
    margin-bottom: 8px;
    font-weight: 500;
    color: var(--color-neutral-700);
}

.topic-selector__select {
    width: 100%;
    padding: 12px 16px;
    border: 1px solid var(--color-neutral-300);
    border-radius: 8px;
    font-size: 1rem;
    background-color: #ffffff;
    color: var(--color-neutral-900);
    cursor: pointer;
    transition: border-color 0.2s ease;
}

.topic-selector__select:focus {
    outline: none;
    border-color: var(--color-primary);
    box-shadow: 0 0 0 3px rgba(37, 99, 235, 0.1);
}

.btn {
    display: inline-block;
    padding: 12px 24px;
    font-size: 1rem;
    font-weight: 500;
    text-decoration: none;
    border: none;
    border-radius: 8px;
    cursor: pointer;
    transition: all 0.2s ease;
    margin: 8px;
    min-width: 120px;
}

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

.btn--primary:hover:not(:disabled) {
    background-color: var(--color-primary-dark);
    transform: translateY(-1px);
}

.btn--primary:disabled {
    background-color: var(--color-neutral-400);
    cursor: not-allowed;
    transform: none;
}

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

.btn--secondary:hover:not(:disabled) {
    background-color: var(--color-neutral-100);
}

.btn:focus {
    outline: none;
    box-shadow: 0 0 0 3px rgba(37, 99, 235, 0.3);
}

.quiz-header {
    display: grid;
    grid-template-columns: 1fr auto auto;
    gap: 24px;
    align-items: center;
    margin-bottom: 24px;
    padding: 16px;
    background-color: #ffffff;
    border: 1px solid var(--color-neutral-200);
    border-radius: 8px;
}

.quiz-progress__text {
    font-size: 0.875rem;
    color: var(--color-neutral-600);
    margin-bottom: 8px;
    display: block;
}

.quiz-progress__bar {
    width: 200px;
    height: 8px;
    background-color: var(--color-neutral-200);
    border-radius: 4px;
    overflow: hidden;
}

.quiz-progress__fill {
    height: 100%;
    background-color: var(--color-primary);
    transition: width 0.3s ease;
    border-radius: 4px;
}

.quiz-timer,
.quiz-score {
    text-align: center;
    font-size: 0.875rem;
}

.quiz-timer__label,
.quiz-score__label {
    display: block;
    color: var(--color-neutral-500);
    margin-bottom: 4px;
}

.quiz-timer__value,
.quiz-score__value {
    font-size: 1.25rem;
    font-weight: 600;
    color: var(--color-neutral-900);
}

.quiz-timer__value--warning {
    color: var(--color-error);
    animation: pulse 1s infinite;
}

@keyframes pulse {
    0%, 100% { opacity: 1; }
    50% { opacity: 0.7; }
}

.question-card {
    background-color: #ffffff;
    border: 1px solid var(--color-neutral-200);
    border-radius: 8px;
    padding: 24px;
    margin-bottom: 24px;
}

.question-card__title {
    margin-bottom: 24px;
    color: var(--color-neutral-900);
    font-size: 1.25rem;
    line-height: 1.5;
}

.question__difficulty {
    display: inline-block;
    font-size: 0.75rem;
    font-weight: 500;
    padding: 4px 8px;
    border-radius: 4px;
    margin-left: 12px;
    background-color: var(--color-neutral-200);
    color: var(--color-neutral-700);
}

.question__difficulty[data-difficulty="1"],
.question__difficulty[data-difficulty="2"] {
    background-color: #dcfce7;
    color: var(--color-success);
}

.question__difficulty[data-difficulty="3"] {
    background-color: #fef3c7;
    color: var(--color-warning);
}

.question__difficulty[data-difficulty="4"],
.question__difficulty[data-difficulty="5"] {
    background-color: #fee2e2;
    color: var(--color-error);
}

.answers {
    margin-bottom: 24px;
}

.answer {
    margin-bottom: 12px;
}

.answer__input {
    position: absolute;
    opacity: 0;
    pointer-events: none;
}

.answer__label {
    display: flex;
    align-items: center;
    padding: 16px;
    border: 1px solid var(--color-neutral-300);
    border-radius: 8px;
    cursor: pointer;
    transition: all 0.2s ease;
    background-color: #ffffff;
}

.answer__label:hover {
    border-color: var(--color-primary);
    background-color: var(--color-neutral-50);
}

.answer--selected .answer__label {
    border-color: var(--color-primary);
    background-color: #eff6ff;
}

.answer--correct .answer__label {
    border-color: var(--color-success);
    background-color: #ecfdf5;
    color: var(--color-success);
}

.answer--incorrect .answer__label {
    border-color: var(--color-error);
    background-color: #fef2f2;
    color: var(--color-error);
}

.answer__letter {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    width: 32px;
    height: 32px;
    background-color: var(--color-neutral-200);
    border-radius: 50%;
    font-weight: 600;
    font-size: 0.875rem;
    margin-right: 16px;
    flex-shrink: 0;
    color: var(--color-neutral-700);
}

.answer--selected .answer__letter {
    background-color: var(--color-primary);
    color: #ffffff;
}

.answer--correct .answer__letter {
    background-color: var(--color-success);
    color: #ffffff;
}

.answer--incorrect .answer__letter {
    background-color: var(--color-error);
    color: #ffffff;
}

.answer__text {
    flex: 1;
    line-height: 1.5;
}

.question-actions {
    display: flex;
    justify-content: space-between;
    gap: 16px;
}

.feedback {
    background-color: #ffffff;
    border: 1px solid var(--color-neutral-200);
    border-radius: 8px;
    padding: 24px;
    text-align: center;
}

.feedback--correct {
    border-color: var(--color-success);
    background-color: #ecfdf5;
}

.feedback--incorrect {
    border-color: var(--color-error);
    background-color: #fef2f2;
}

.feedback__message {
    margin-bottom: 16px;
    line-height: 1.6;
}

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

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

.screen--results {
    text-align: center;
    max-width: 500px;
    margin: 0 auto;
    padding-top: 48px;
}

.results-summary {
    background-color: #ffffff;
    border: 1px solid var(--color-neutral-200);
    border-radius: 8px;
    padding: 32px 24px;
    margin-bottom: 24px;
}

.results-summary__score {
    margin-bottom: 16px;
}

.results-summary__label {
    display: block;
    color: var(--color-neutral-600);
    font-size: 0.875rem;
    margin-bottom: 8px;
}

.results-summary__value {
    font-size: 3rem;
    font-weight: 700;
    color: var(--color-primary);
    line-height: 1;
}

.results-summary__total {
    color: var(--color-neutral-600);
    font-size: 1rem;
    margin-left: 8px;
}

.results-summary__percentage {
    font-size: 1.5rem;
    font-weight: 600;
    color: var(--color-success);
    margin-bottom: 16px;
}

.results-summary__message {
    color: var(--color-neutral-700);
    font-weight: 500;
}

.results-actions {
    display: flex;
    flex-direction: column;
    gap: 8px;
    align-items: center;
}

.screen--leaderboard {
    max-width: 600px;
    margin: 0 auto;
}

.leaderboard-filters {
    margin-bottom: 24px;
}

.leaderboard-filters__label {
    display: block;
    margin-bottom: 8px;
    font-weight: 500;
    color: var(--color-neutral-700);
}

.leaderboard-filters__select {
    width: 100%;
    max-width: 300px;
    padding: 12px 16px;
    border: 1px solid var(--color-neutral-300);
    border-radius: 8px;
    font-size: 1rem;
    background-color: #ffffff;
    color: var(--color-neutral-900);
    cursor: pointer;
    transition: border-color 0.2s ease;
}

.leaderboard-filters__select:focus {
    outline: none;
    border-color: var(--color-primary);
    box-shadow: 0 0 0 3px rgba(37, 99, 235, 0.1);
}

.leaderboard {
    background-color: #ffffff;
    border: 1px solid var(--color-neutral-200);
    border-radius: 8px;
    padding: 24px;
    margin-bottom: 24px;
}

.leaderboard__empty {
    text-align: center;
    color: var(--color-neutral-500);
    font-style: italic;
}

.leaderboard__entry {
    display: flex;
    align-items: center;
    padding: 16px 0;
    border-bottom: 1px solid var(--color-neutral-200);
}

.leaderboard__entry:last-child {
    border-bottom: none;
}

.leaderboard__rank {
    width: 40px;
    height: 40px;
    background-color: var(--color-primary);
    color: #ffffff;
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
    font-weight: 600;
    margin-right: 16px;
    flex-shrink: 0;
}

.leaderboard__entry:first-child .leaderboard__rank {
    background-color: var(--color-warning);
}

.leaderboard__entry:nth-child(2) .leaderboard__rank {
    background-color: var(--color-neutral-500);
}

.leaderboard__entry:nth-child(3) .leaderboard__rank {
    background-color: #cd7c2f;
}

.leaderboard__details {
    flex: 1;
}

.leaderboard__score {
    font-size: 1.25rem;
    font-weight: 600;
    color: var(--color-neutral-900);
    margin-bottom: 4px;
}

.leaderboard__meta {
    display: flex;
    gap: 16px;
    font-size: 0.875rem;
    color: var(--color-neutral-500);
}

.toast {
    position: fixed;
    top: 24px;
    right: 24px;
    max-width: 400px;
    z-index: 1000;
    animation: slideIn 0.3s ease-out;
}

.toast--error {
    background-color: #fef2f2;
    border: 1px solid #fecaca;
    color: var(--color-error);
}

.toast__content {
    padding: 16px;
    border-radius: 8px;
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 16px;
}

.toast__message {
    flex: 1;
    line-height: 1.5;
}

.toast__close {
    background: none;
    border: none;
    color: inherit;
    font-size: 1.5rem;
    cursor: pointer;
    padding: 0;
    width: 24px;
    height: 24px;
    display: flex;
    align-items: center;
    justify-content: center;
    border-radius: 4px;
    transition: background-color 0.2s ease;
}

.toast__close:hover {
    background-color: rgba(0, 0, 0, 0.1);
}

@keyframes slideIn {
    from {
        transform: translateX(100%);
        opacity: 0;
    }
    to {
        transform: translateX(0);
        opacity: 1;
    }
}

@media (max-width: 768px) {
    .main {
        padding: 16px;
    }
    
    .quiz-header {
        grid-template-columns: 1fr;
        gap: 16px;
        text-align: center;
    }
    
    .quiz-progress__bar {
        width: 100%;
    }
    
    .question-actions {
        flex-direction: column;
    }
    
    .results-actions {
        width: 100%;
    }
    
    .btn {
        width: 100%;
        margin: 4px 0;
    }
    
    .leaderboard__meta {
        flex-direction: column;
        gap: 4px;
    }
    
    .toast {
        left: 16px;
        right: 16px;
        max-width: none;
    }
}

@media (max-width: 480px) {
    .header {
        padding: 16px;
    }
    
    .header__title {
        font-size: 1.5rem;
    }
    
    .question-card {
        padding: 16px;
    }
    
    .question-card__title {
        font-size: 1.125rem;
    }
    
    .results-summary {
        padding: 24px 16px;
    }
    
    .results-summary__value {
        font-size: 2.5rem;
    }
}

.btn:focus,
.answer__label:focus-within,
.topic-selector__select:focus,
.leaderboard-filters__select:focus {
    outline: none;
    box-shadow: 0 0 0 3px rgba(37, 99, 235, 0.3);
}

@media (prefers-reduced-motion: reduce) {
    *,
    *::before,
    *::after {
        animation-duration: 0.01ms !important;
        animation-iteration-count: 1 !important;
        transition-duration: 0.01ms !important;
    }
}