:root {
    --bg-main: #0f172a;
    --bg-sidebar: #1e293b;
    --bg-card: #1e293b;
    --bg-input: #0f172a;
    --border-color: #334155;
    
    --text-primary: #f1f5f9;
    --text-secondary: #94a3b8;
    --text-muted: #64748b;
    
    --accent-blue: #2563eb;
    --accent-blue-hover: #1d4ed8;
    --accent-green: #16a34a;
    --accent-red: #ef4444;
    --accent-yellow: #eab308;
    --ai-purple: #a855f7;
    
    --syn-keyword: #c678dd;
    --syn-func: #61afef;
    --syn-string: #98c379;
    --syn-comment: #5c6370;
    --syn-number: #d19a66;
}

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

body {
    font-family: 'Inter', -apple-system, sans-serif;
    background-color: var(--bg-main);
    color: var(--text-primary);
    height: 100vh;
    overflow: hidden;
}

.app-layout {
    display: grid;
    grid-template-columns: 18rem 1fr;
    height: 100vh;
    overflow: hidden;
}

/* Sidebar Navigation */
.sidebar {
    background-color: var(--bg-sidebar);
    border-right: 1px solid var(--border-color);
    display: flex;
    flex-direction: column;
    overflow-y: auto;
    scrollbar-gutter: stable;
    transition: left 0.3s cubic-bezier(0.4, 0, 0.2, 1);
}

.sidebar-header {
    padding: 1.5rem;
    border-bottom: 1px solid var(--border-color);
    font-weight: 700;
    font-size: 1.25rem;
    color: white;
    display: flex;
    justify-content: space-between;
    align-items: center;
}

.lesson-btn {
    width: 100%;
    text-align: left;
    padding: 1rem 1.5rem;
    background: none;
    border: none;
    border-left: 4px solid transparent;
    color: var(--text-secondary);
    cursor: pointer;
    transition: 0.2s;
}

.lesson-btn:hover { background: rgba(255,255,255,0.05); }
.lesson-btn.active {
    background: rgba(255,255,255,0.08);
    border-left-color: var(--accent-blue);
    color: white;
}

/* Main Workspace */
.main-content {
    background-color: var(--bg-main);
    overflow-y: auto;
    scrollbar-gutter: stable;
    padding: 2rem;
    display: flex;
    flex-direction: column;
    
}

.container {
    max-width: 64rem;
    margin: 0 auto;
    width: 100%;
    flex: 1;
}

/* UI Cards */
.card {
    background-color: var(--bg-card);
    border: 1px solid var(--border-color);
    border-radius: 0.75rem;
    margin-bottom: 2rem;
    overflow: hidden;
}

.card-header {
    background: rgba(0,0,0,0.2);
    padding: 1rem;
    border-bottom: 1px solid var(--border-color);
    display: flex;
    justify-content: space-between;
    align-items: center;
}

/* AI & Meta */
.ai-badge {
    background: linear-gradient(135deg, var(--accent-blue) 0%, var(--ai-purple) 100%);
    color: white;
    padding: 0.25rem 0.75rem;
    border-radius: 99px;
    font-size: 0.75rem;
    font-weight: 600;
    cursor: pointer;
    border: none;
}

.ai-box {
    background: rgba(168, 85, 247, 0.1);
    border-left: 4px solid var(--ai-purple);
    padding: 1.25rem;
    margin: 1rem 0;
    border-radius: 0 0.5rem 0.5rem 0;
    line-height: 1.6;
}

/* Code Syntax & Editor */
.editor {
    width: 100%;
    background: var(--bg-input);
    color: var(--text-primary);
    padding: 1rem;
    font-family: 'Fira Code', monospace;
    font-size: 14px;
    border: 1px solid var(--border-color);
    border-radius: 0.5rem;
    resize: none;
    outline: none;
    overflow: hidden; /* Remove internal scroll */
}

.btn {
    padding: 0.6rem 1.25rem;
    border-radius: 0.5rem;
    font-weight: 600;
    cursor: pointer;
    border: none;
    transition: 0.2s;
}

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

/* Responsive Breakpoints */
.mobile-header { display: none; margin-bottom: 2rem; align-items: center; gap: 1rem; padding-bottom: 1rem; border-bottom: 1px solid var(--border-color); }
.menu-toggle { background: #1e293b; border: 1px solid var(--border-color); color: #fff; padding: 0.5rem; border-radius: 8px; cursor: pointer; display: flex; align-items: center; }
.mobile-close-btn { display: none; background: transparent; border: none; color: #fff; cursor: pointer; }

/* Trigger mobile view only below 768px. Exactly 768px remains desktop view. */
@media (max-width: 767px) {
    .app-layout { grid-template-columns: 1fr; }
    .mobile-header { display: flex; }
    .sidebar { position: fixed; left: -100%; top: 0; bottom: 0; z-index: 1000; width: 280px; box-shadow: 10px 0 30px rgba(0,0,0,0.5); }
    .sidebar.mobile-open { left: 0; }
    .mobile-close-btn { display: block; }
    .main-content { padding: 1.5rem 1rem; }
}

/* Footer */
.main-footer { margin-top: auto; padding: 3rem 0 2rem 0; border-top: 1px solid var(--border-color); text-align: center; }
.contact-message { color: var(--text-secondary); font-size: 0.9rem; margin-bottom: 0.5rem; }
.contact-message a { color: var(--accent-blue); text-decoration: none; font-weight: 700; }

/* Syntax Highlighting */
.kwd { color: var(--syn-keyword); }
.fn { color: var(--syn-func); }
.str { color: var(--syn-string); }
.com { color: var(--syn-comment); }
.num { color: var(--syn-number); }

::-webkit-scrollbar { width: 8px; }
::-webkit-scrollbar-thumb { background: #475569; border-radius: 4px; }