/* ===== ROOT VARIABLES ===== */
:root{
  --accent: #4f46e5;
  --bg: #fefefe;
  --fg: #2d3748;
  --panel: #ffffff;
  --panel-border: #e2e8f0;
  --code-bg: #f7fafc;
  --hover: #f1f5f9;
  --shadow: rgba(0,0,0,0.1);
  --success: #48bb78;
}

[data-theme='dark']{
  --accent: #818cf8;
  --bg: #1a202c;
  --fg: #f7fafc;
  --panel: #2d3748;
  --panel-border: #4a5568;
  --code-bg: #2d3748;
  --hover: #4a5568;
  --shadow: rgba(0,0,0,0.3);
}

/* ===== GLOBAL STYLES ===== */
*{box-sizing:border-box}
body{margin:0;display:flex;background:var(--bg);color:var(--fg);font:16px/1.6 system-ui, sans-serif;transition:background-color 0.3s, color 0.3s}

/* ===== MODERN SIDEBAR NAVIGATION ===== */
.sidebar{
  width: 280px;
  height: 100vh;
  max-height: 100vh;
  background: var(--panel);
  border-right: 1px solid var(--panel-border);
  position: fixed;
  left: 0;
  top: 0;
  overflow-y: auto;
  overflow-x: hidden;
  padding: 20px 0;
  z-index: 100;
  /* Ensure smooth scrolling */
  scroll-behavior: smooth;
  /* Better scrollbar styling */
  scrollbar-width: thin;
  scrollbar-color: var(--accent) transparent;
}

/* Webkit scrollbar styling */
.sidebar::-webkit-scrollbar {
  width: 6px;
}

.sidebar::-webkit-scrollbar-track {
  background: transparent;
}

.sidebar::-webkit-scrollbar-thumb {
  background: var(--accent);
  border-radius: 3px;
}

.sidebar::-webkit-scrollbar-thumb:hover {
  background: var(--fg);
}

/* Navigation List - Clean, Flat Structure */
.sidebar-nav{
  list-style: none;
  padding: 0;
  margin: 0;
}

/* Section Headers - Prominent but not clickable */
.section-header {
  font-size: 14px;
  font-weight: 600;
  color: var(--accent);
  text-transform: uppercase;
  letter-spacing: 0.5px;
  padding: 16px 24px 8px;
  margin: 16px 0 0 0;
  border-bottom: 1px solid var(--panel-border);
}

.section-header:first-child {
  margin-top: 0;
}

/* Navigation Links - Simple, Always Visible */
.nav-link {
  display: block;
  padding: 12px 24px;
  color: var(--fg);
  text-decoration: none;
  border-left: 3px solid transparent;
  transition: all 0.2s ease;
  font-size: 15px;
  line-height: 1.4;
}

.nav-link:hover {
  background: var(--hover);
  border-left-color: var(--accent);
  transform: translateX(2px);
}

/* Active/Current Page Styling */
.nav-link.active {
  background: var(--hover);
  border-left-color: var(--accent);
  color: var(--accent);
  font-weight: 500;
}

/* Subsection Links - Slightly Indented */
.nav-link.subsection {
  padding-left: 40px;
  font-size: 14px;
  color: rgba(var(--fg), 0.8);
}

.nav-link.subsection:hover {
  color: var(--fg);
}

/* ===== MAIN CONTENT AREA ===== */
.content{
  margin-left: 280px;
  padding: 40px;
  width: calc(100% - 280px);
  background: var(--bg);
  min-height: 100vh;
  transition: background-color 0.3s;
}

.content h1,.content h2{
  color: var(--accent);
  border-bottom: 2px solid var(--panel-border);
  padding-bottom: 12px;
  margin-top: 0;
}

/* ===== FORMULA CARDS ===== */
.formula-card-container{display:flex;flex-direction:column;gap:24px}
.formula-card{
  background: var(--panel);
  border: 1px solid var(--panel-border);
  border-radius: 12px;
  padding: 24px;
  position: relative;
  transition: all 0.3s ease;
  box-shadow: 0 2px 8px var(--shadow);
}
.formula-card:hover{transform:translateY(-2px);box-shadow:0 8px 25px var(--shadow)}

.copy-btn{
  position: absolute;
  top: 16px;
  right: 16px;
  background: var(--accent);
  color: white;
  border: none;
  border-radius: 6px;
  padding: 8px 12px;
  cursor: pointer;
  opacity: 0.7;
  transition: all 0.2s ease;
  font-size: 12px;
}
.formula-card:hover .copy-btn{opacity:1}
.copy-btn:hover{transform:scale(1.05)}
.copy-btn.copied{background:var(--success);animation:pulse 0.6s}

.formula-card h3{margin:0 0 12px;color:var(--accent);font-size:1.3em}
.mathjax-expression{
  font-size: 1.2em;
  margin: 16px 0;
  padding: 16px;
  background: var(--code-bg);
  border-radius: 8px;
}
.variables{list-style:none;padding:0;margin:12px 0}
.variables li{margin-bottom:6px;color:var(--fg);font-family:monospace}
.description{margin-top:12px;line-height:1.7}

.visual-placeholder{
  margin-top: 16px;
  padding: 12px;
  background: var(--code-bg);
  border-radius: 6px;
  font-style: italic;
  color: rgba(var(--fg), 0.6);
}

.derivation{margin-top:16px}
.derivation summary{cursor:pointer;font-weight:600;color:var(--accent);padding:8px 0}
.derivation-content{margin-top:12px;padding:16px;background:var(--code-bg);border-radius:8px;border-left:4px solid var(--accent)}

/* ===== MOBILE HEADER ===== */
.mobile-header{
  display: none;
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  height: 60px;
  background: var(--panel);
  border-bottom: 1px solid var(--panel-border);
  z-index: 101;
  padding: 0 20px;
  align-items: center;
}

#hamburger-menu,#theme-toggle{
  background: none;
  border: none;
  font-size: 18px;
  padding: 8px;
  border-radius: 6px;
  cursor: pointer;
  color: var(--fg);
}
#hamburger-menu:hover,#theme-toggle:hover{background:var(--hover)}
#theme-toggle{margin-left:auto}

/* ===== SEARCH BOX ===== */
.search-box{
  position: fixed;
  top: 20px;
  right: 20px;
  width: 300px;
  padding: 12px 16px;
  border: 2px solid var(--panel-border);
  border-radius: 8px;
  background: var(--panel);
  color: var(--fg);
  font-size: 14px;
  z-index: 1000;
}
.search-box:focus{box-shadow:0 8px 35px var(--shadow), 0 0 0 3px rgba(79, 70, 229, 0.1)}

/* ===== MOBILE RESPONSIVE ===== */
@media(max-width:768px){
  body{flex-direction:column}
  .sidebar{
    position: fixed;
    left: -280px;
    width: 280px;
    height: 100vh;
    max-height: 100vh;
    transition: left 0.3s ease;
    overflow-y: auto;
    padding-top: 70px; /* Account for mobile header */
  }
  .sidebar.active{left:0}
  .content{
    width: 100%;
    margin-left: 0;
    padding: 20px;
    padding-top: 80px;
  }
  .mobile-header{
    display: flex;
    align-items: center;
    justify-content: space-between;
  }
  .search-box{right:10px;width:calc(100vw - 20px);max-width:300px}
  .formula-card{padding:16px}
  .copy-btn{position:static;opacity:1;margin-bottom:12px;align-self:flex-start}
  
  /* Ensure sidebar can scroll on mobile */
  .sidebar-nav {
    padding-bottom: 20px;
  }
  
  /* Better touch targets on mobile */
  .nav-link {
    padding: 16px 24px;
    font-size: 16px;
  }
  
  .section-header {
    padding: 20px 24px 12px;
    font-size: 15px;
  }
}

/* ===== ANIMATIONS ===== */
@keyframes fadeIn{from{opacity:0;transform:translateY(12px)}to{opacity:1;transform:none}}
@keyframes pulse{0%,100%{transform:scale(1)}50%{transform:scale(1.05)}}
@keyframes slideDown{
  from{opacity:0;max-height:0}
  to{opacity:1;max-height:200px}
}

/* ===== ACCESSIBILITY ===== */
.sr-only{position:absolute;width:1px;height:1px;padding:0;margin:-1px;overflow:hidden;clip:rect(0,0,0,0);white-space:nowrap;border:0}

/* ===== PRINT STYLES ===== */
@media print{
  .sidebar,.mobile-header,.copy-btn,.search-box{display:none!important}
  .content{margin-left:0;width:100%;padding:20px}
  .formula-card{break-inside:avoid;margin-bottom:20px;box-shadow:none;border:1px solid #ddd}
}