* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

:root {
    --bg: #0d1117;
    --surface: #161b22;
    --border: #30363d;
    --text: #e6edf3;
    --text-muted: #8b949e;
    --accent: #58a6ff;
    --accent-hover: #79c0ff;
    --green: #3fb950;
    --red: #f85149;
}

body {
    font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif;
    background: var(--bg);
    color: var(--text);
    line-height: 1.6;
}

header {
    border-bottom: 1px solid var(--border);
    padding: 1rem 2rem;
    position: sticky;
    top: 0;
    background: var(--bg);
    z-index: 100;
}

nav {
    max-width: 1000px;
    margin: 0 auto;
    display: flex;
    justify-content: space-between;
    align-items: center;
}

.logo {
    font-size: 1.25rem;
    font-weight: 700;
    color: var(--accent);
}

nav ul {
    list-style: none;
    display: flex;
    gap: 1.5rem;
}

nav a {
    color: var(--text-muted);
    text-decoration: none;
    transition: color 0.2s;
}

nav a:hover {
    color: var(--text);
}

main {
    max-width: 1000px;
    margin: 0 auto;
    padding: 2rem;
}

#hero {
    text-align: center;
    padding: 3rem 0;
}

#hero h1 {
    font-size: 2.5rem;
    margin-bottom: 0.5rem;
}

#hero p {
    color: var(--text-muted);
    font-size: 1.2rem;
}

h2 {
    margin-bottom: 1rem;
    padding-bottom: 0.5rem;
    border-bottom: 1px solid var(--border);
}

/* Editor */
#editor-container {
    border: 1px solid var(--border);
    border-radius: 8px;
    overflow: hidden;
}

.CodeMirror {
    height: auto;
    min-height: 120px;
    max-height: 400px;
    font-size: 15px;
    line-height: 1.5;
}

#controls {
    display: flex;
    gap: 0.5rem;
    margin: 0.75rem 0;
}

button {
    padding: 0.5rem 1.25rem;
    border: 1px solid var(--border);
    border-radius: 6px;
    background: var(--surface);
    color: var(--text);
    cursor: pointer;
    font-size: 0.9rem;
    transition: background 0.2s, border-color 0.2s;
}

button:hover:not(:disabled) {
    background: var(--border);
}

#run-btn {
    background: #238636;
    border-color: #2ea043;
}

#run-btn:hover:not(:disabled) {
    background: #2ea043;
}

#run-btn:disabled {
    opacity: 0.6;
    cursor: wait;
}

/* Output */
#output-container {
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: 8px;
    padding: 1rem;
    min-height: 60px;
    max-height: 300px;
    overflow-y: auto;
    margin-bottom: 3rem;
}

#output {
    font-family: 'SF Mono', 'Fira Code', 'Fira Mono', Menlo, Consolas, monospace;
    font-size: 14px;
    white-space: pre-wrap;
    word-break: break-word;
    color: var(--text);
}

.output-error {
    color: var(--red) !important;
}

/* Problems */
#problems {
    margin-top: 2rem;
}

.problem-card {
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: 8px;
    padding: 1.25rem;
    margin-bottom: 1rem;
    cursor: pointer;
    transition: border-color 0.2s;
}

.problem-card:hover {
    border-color: var(--accent);
}

.problem-card h3 {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 0.5rem;
}

.problem-card p {
    color: var(--text-muted);
    font-size: 0.95rem;
}

.difficulty {
    font-size: 0.75rem;
    padding: 0.2rem 0.6rem;
    border-radius: 12px;
    font-weight: 600;
}

.difficulty.easy {
    background: rgba(63, 185, 80, 0.15);
    color: var(--green);
}

.difficulty.medium {
    background: rgba(210, 153, 34, 0.15);
    color: #d29922;
}

.difficulty.hard {
    background: rgba(248, 81, 73, 0.15);
    color: var(--red);
}

.problem-active {
    border-color: var(--accent);
}

.problem-detail {
    margin-top: 1rem;
    padding-top: 1rem;
    border-top: 1px solid var(--border);
    display: none;
}

.problem-active .problem-detail {
    display: block;
}

.problem-detail pre {
    background: var(--bg);
    padding: 0.75rem;
    border-radius: 6px;
    font-size: 14px;
    margin: 0.5rem 0;
    overflow-x: auto;
}

.problem-detail button {
    margin-top: 0.5rem;
}

.load-btn {
    background: var(--accent);
    border-color: var(--accent);
    color: #000;
    font-weight: 600;
}

.load-btn:hover {
    background: var(--accent-hover);
}

footer {
    text-align: center;
    padding: 2rem;
    color: var(--text-muted);
    border-top: 1px solid var(--border);
    margin-top: 2rem;
}