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

:root {
  --bg-color: #f8fafc;
  /* Slightly off-white for depth */
  --text-color: #1e293b;
  --text-muted: #64748b;
  --border-color: #e2e8f0;
  --link-color: #2563eb;
  --header-bg: #ffffff;
  --code-bg: #f1f5f9;

  --card-bg: #ffffff;
  --card-hover-bg: #ffffff;

  --tag-bg: #e0f2fe;
  --tag-text: #0284c7;

  /* Difficulty Levels (5 levels) - Refined Palette */
  --diff-basic-color: #475569;
  --diff-basic-bg: #f1f5f9;
  --diff-easy-color: #15803d;
  --diff-easy-bg: #dcfce7;
  --diff-medium-color: #a16207;
  --diff-medium-bg: #fef9c3;
  --diff-hard-color: #b91c1c;
  --diff-hard-bg: #fee2e2;
  --diff-expert-color: #7e22ce;
  --diff-expert-bg: #f3e8ff;

  /* Category Colors */
  --cat-dsa-color: #2563eb;
  --cat-sys-color: #10b981;
  --cat-db-color: #f59e0b;
  --cat-js-color: #eab308;
  --cat-lld-color: #8b5cf6;
  --cat-dsa-oop-color: #ec4899;

  /* Platform Colors */
  --source-leetcode-color: #ea580c;
  --source-leetcode-bg: #fff7ed;
  --source-hackerrank-color: #15803d;
  --source-hackerrank-bg: #dcfce7;
  --source-neetcode-color: #1d4ed8;
  --source-neetcode-bg: #dbeafe;
  --source-other-color: #475569;
  --source-other-bg: #f1f5f9;

  --shadow-sm: 0 1px 2px 0 rgb(0 0 0 / 0.05);
  --shadow-md: 0 4px 6px -1px rgb(0 0 0 / 0.1), 0 2px 4px -2px rgb(0 0 0 / 0.1);
  --shadow-lg: 0 10px 15px -3px rgb(0 0 0 / 0.1), 0 4px 6px -4px rgb(0 0 0 / 0.1);
}

[data-theme="dark"] {
  --bg-color: #0f172a;
  --text-color: #e2e8f0;
  --text-muted: #94a3b8;
  --border-color: #1e293b;
  --link-color: #38bdf8;
  --header-bg: #1e293b;
  --code-bg: #1e293b;

  --card-bg: #1e293b;
  --card-hover-bg: #334155;

  --tag-bg: #0c4a6e;
  --tag-text: #38bdf8;

  --diff-basic-bg: rgba(71, 85, 105, 0.3);
  --diff-basic-color: #cbd5e1;
  --diff-easy-bg: rgba(21, 128, 61, 0.3);
  --diff-easy-color: #86efac;
  --diff-medium-bg: rgba(161, 98, 7, 0.3);
  --diff-medium-color: #fde047;
  --diff-hard-bg: rgba(185, 28, 28, 0.3);
  --diff-hard-color: #fca5a5;
  --diff-expert-bg: rgba(126, 34, 206, 0.3);
  --diff-expert-color: #d8b4fe;

  --source-leetcode-bg: rgba(234, 88, 12, 0.25);
  --source-leetcode-color: #fdba74;
  --source-hackerrank-bg: rgba(21, 128, 61, 0.25);
  --source-hackerrank-color: #86efac;
  --source-neetcode-bg: rgba(29, 78, 216, 0.3);
  --source-neetcode-color: #93c5fd;
  --source-other-bg: rgba(71, 85, 105, 0.35);
  --source-other-color: #cbd5e1;
}

body {
  font-family: 'Inter', -apple-system, BlinkMacSystemFont, "Segoe UI", Helvetica, Arial, sans-serif;
  line-height: 1.6;
  color: var(--text-color);
  background-color: var(--bg-color);
  margin: 0;
  padding: 0;
  transition: background-color 0.3s, color 0.3s;
}

.container {
  max-width: 1024px;
  margin: 0 auto;
  padding: 0 1.5rem;
}

/* Header */
header {
  background-color: var(--header-bg);
  border-bottom: 1px solid var(--border-color);
  padding: 1rem 0;
  margin-bottom: 2rem;
  position: sticky;
  top: 0;
  z-index: 100;
  backdrop-filter: blur(8px);
  background: rgba(255, 255, 255, 0.8);
}

[data-theme="dark"] header {
  background: rgba(30, 41, 59, 0.8);
}

header .container {
  display: flex;
  justify-content: space-between;
  align-items: center;
}

.site-title {
  font-weight: 700;
  font-size: 1.1rem;
  color: var(--text-color);
  text-decoration: none;
}

nav a {
  margin-left: 1.5rem;
  color: var(--text-muted);
  text-decoration: none;
  font-weight: 500;
  font-size: 0.95rem;
  transition: color 0.2s;
}

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

h1,
h2,
h3,
h4,
h5,
h6 {
  margin-top: 0;
  margin-bottom: 1rem;
  font-weight: 700;
  line-height: 1.25;
  color: var(--text-color);
}

a {
  color: var(--link-color);
  text-decoration: none;
}

a:hover {
  text-decoration: underline;
}

/* Hero Section */
.hero-section {
  text-align: center;
  padding: 2rem 1rem 2rem;
}

.hero-section h1 {
  font-size: 2.5rem;
  background: linear-gradient(135deg, var(--cat-dsa-color), var(--cat-lld-color));
  background-clip: text;
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  margin-bottom: 1rem;
}

.hero-section p {
  font-size: 1.2rem;
  color: var(--text-muted);
  max-width: 600px;
  margin: 0 auto;
}

/* Cards */
.card {
  border: 1px solid var(--border-color);
  border-radius: 12px;
  background-color: var(--card-bg);
  padding: 1.5rem;
  margin-bottom: 1.5rem;
  box-shadow: var(--shadow-sm);
}

/* Content Area */
.content {
  line-height: 1.7;
  color: var(--text-color);
}

.content h2 {
  margin-top: 2rem;
  margin-bottom: 1rem;
  padding-bottom: 0.5rem;
  border-bottom: 1px solid var(--border-color);
}

.content h3 {
  margin-top: 1.5rem;
  margin-bottom: 0.75rem;
}

.content p {
  margin-bottom: 1rem;
}

.content ul,
.content ol {
  margin-bottom: 1rem;
  padding-left: 1.5rem;
}

.content li {
  margin-bottom: 0.5rem;
}

.content code {
  background-color: var(--code-bg);
  padding: 0.2em 0.4em;
  border-radius: 4px;
  font-size: 0.9em;
  border: 1px solid var(--border-color);
}

.content pre code {
  background: none;
  padding: 0;
  border: none;
  font-size: inherit;
}

/* Stats Grid */
.stats-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(150px, 1fr));
  gap: 1rem;
  margin-bottom: 2rem;
}

.stat-card {
  text-align: center;
  padding: 1.5rem;
  margin-bottom: 0;
}

.stat-value {
  font-size: 2rem;
  font-weight: 700;
  display: block;
  color: var(--text-color);
}

.stat-label {
  color: var(--text-muted);
  font-size: 0.9rem;
  display: block;
  margin-top: 0.5rem;
}

.platform-stats-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(180px, 1fr));
  gap: 1rem;
  margin-bottom: 2rem;
}

.platform-stat {
  text-align: center;
  padding: 1rem 1.25rem;
  margin-bottom: 0;
}

.platform-stat-value {
  font-size: 1.6rem;
  font-weight: 700;
  display: block;
}

.platform-stat-label {
  color: var(--text-muted);
  font-size: 0.85rem;
  text-transform: uppercase;
  letter-spacing: 0.05em;
}

.platform-leetcode {
  border-bottom: 4px solid var(--source-leetcode-color);
}

.platform-hackerrank {
  border-bottom: 4px solid var(--source-hackerrank-color);
}

.platform-neetcode {
  border-bottom: 4px solid var(--source-neetcode-color);
}

.platform-other {
  border-bottom: 4px solid var(--source-other-color);
}

.platform-section {
  width: 100%;
  margin-bottom: 2rem;
}

.platform-section-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 1rem;
  margin: 0.5rem 0 1rem;
}

.platform-section-header h2 {
  margin: 0;
  font-size: 1.15rem;
}

/* Category Grid */
.category-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(280px, 1fr));
  gap: 1.5rem;
  margin-bottom: 4rem;
}

.category-card {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  padding: 2rem;
  border-radius: 16px;
  text-align: center;
  text-decoration: none !important;
  border: 1px solid var(--border-color);
  background-color: var(--card-bg);
  transition: all 0.3s cubic-bezier(0.4, 0, 0.2, 1);
  position: relative;
  overflow: hidden;
  box-shadow: var(--shadow-sm);
}

.category-card:hover {
  transform: translateY(-8px);
  box-shadow: var(--shadow-lg);
  border-color: var(--link-color);
}

.cat-icon {
  font-size: 2.5rem;
  margin-bottom: 1rem;
  background: var(--bg-color);
  width: 64px;
  height: 64px;
  display: flex;
  align-items: center;
  justify-content: center;
  border-radius: 50%;
  margin-bottom: 1.5rem;
}

.category-card h3 {
  margin: 0 0 0.5rem;
  font-size: 1.1rem;
  font-weight: 600;
}

.category-card p {
  margin: 0 0 1.5rem;
  font-size: 0.9rem;
  color: var(--text-muted);
  line-height: 1.4;
}

/* Category Specific Borders/Accents */
.cat-dsa:hover {
  border-color: var(--cat-dsa-color);
}

.cat-system-design:hover {
  border-color: var(--cat-sys-color);
}

.cat-database:hover {
  border-color: var(--cat-db-color);
}

.cat-javascript:hover {
  border-color: var(--cat-js-color);
}

.cat-lld:hover {
  border-color: var(--cat-lld-color);
}

.cat-dsa-oop:hover {
  border-color: var(--cat-dsa-oop-color);
}


/* Problem List Cards */
.section-title {
  margin-top: 6rem;
  margin-bottom: 1.5rem;
  font-size: 1.5rem;
  display: flex;
  align-items: center;
  gap: 0.5rem;
}

.problem-list {
  display: flex;
  flex-direction: column;
  gap: 1rem;
}

.problem-card {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: 1.25rem 1.5rem;
  background: var(--card-bg);
  border: 1px solid var(--border-color);
  border-radius: 12px;
  text-decoration: none !important;
  color: var(--text-color);
  transition: all 0.2s ease;
  box-shadow: var(--shadow-sm);
}

.problem-card:hover {
  border-color: var(--link-color);
  transform: translateX(4px);
  box-shadow: var(--shadow-md);
}

.problem-info h3 {
  margin: 0 0 0.5rem 0;
  font-size: 1.05rem;
  font-weight: 600;
  color: var(--text-color);
}

.problem-card:hover .problem-info h3 {
  color: var(--link-color);
}

.problem-meta {
  display: flex;
  gap: 0.75rem;
  font-size: 0.8rem;
  color: var(--text-muted);
  align-items: center;
  flex-wrap: wrap;
}

/* Badges */
.badge {
  display: inline-block;
  padding: 0.25em 0.6em;
  font-size: 0.7rem;
  font-weight: 600;
  line-height: 1;
  border-radius: 6px;
  border: 1px solid transparent;
  text-transform: uppercase;
  letter-spacing: 0.05em;
}

.category-header {
  padding: 3rem 2rem;
  margin-bottom: 2rem;
  border-radius: 16px;
  background-color: var(--card-bg);
  border: 1px solid var(--border-color);
  text-align: center;
  box-shadow: var(--shadow-sm);
}

/* Category Header Themes */
.category-dsa h1 {
  color: var(--cat-dsa-color);
}

.category-system-design h1 {
  color: var(--cat-sys-color);
}

.category-database h1 {
  color: var(--cat-db-color);
}

.category-javascript h1 {
  color: var(--cat-js-color);
}

.category-lld h1 {
  color: var(--cat-lld-color);
}

.category-dsa-oop h1 {
  color: var(--cat-dsa-oop-color);
}


/* Difficulty Badges */
.badge-difficulty-basic {
  color: var(--diff-basic-color);
  background: var(--diff-basic-bg);
}


.badge-difficulty-easy {
  color: var(--diff-easy-color);
  background: var(--diff-easy-bg);
}


.badge-difficulty-medium {
  color: var(--diff-medium-color);
  background: var(--diff-medium-bg);
}

.badge-difficulty-hard {
  color: var(--diff-hard-color);
  background: var(--diff-hard-bg);
}

.badge-difficulty-expert {
  color: var(--diff-expert-color);
  background: var(--diff-expert-bg);
}

.badge-category {
  background: var(--bg-color);
  border: 1px solid var(--border-color);
  color: var(--text-color);
}

.badge-source {
  border: 1px solid transparent;
  text-transform: none;
  letter-spacing: normal;
}

.badge-source-leetcode {
  color: var(--source-leetcode-color);
  background: var(--source-leetcode-bg);
}

.badge-source-hackerrank {
  color: var(--source-hackerrank-color);
  background: var(--source-hackerrank-bg);
}

.badge-source-neetcode {
  color: var(--source-neetcode-color);
  background: var(--source-neetcode-bg);
}

.badge-source-other {
  color: var(--source-other-color);
  background: var(--source-other-bg);
}

.tag {
  background-color: var(--tag-bg);
  color: var(--tag-text);
  border-radius: 4px;
  padding: 2px 8px;
  font-size: 0.75rem;
  font-weight: 500;
  display: inline-block;
}

/* Buttons */
.button-ghost {
  display: inline-block;
  padding: 0.5rem 1rem;
  color: var(--text-muted);
  border: 1px solid var(--border-color);
  border-radius: 6px;
  font-size: 0.9rem;
  font-weight: 500;
  transition: all 0.2s;
}

.button-ghost:hover {
  color: var(--text-color);
  border-color: var(--text-muted);
  background: var(--bg-color);
  text-decoration: none;
}

/* Code */
pre {
  background-color: var(--code-bg);
  padding: 1.5rem;
  border-radius: 12px;
  overflow: auto;
  border: 1px solid var(--border-color);
  font-size: 0.9rem;
}

code {
  font-family: ui-monospace, SFMono-Regular, SF Mono, Menlo, Consolas, Liberation Mono, monospace;
}

/* Table Styling for Archive Page */
table {
  width: 100%;
  border-collapse: collapse;
  font-size: 0.95rem;
}

table thead {
  background-color: var(--bg-color);
  border-bottom: 2px solid var(--border-color);
}

table th {
  padding: 0.75rem 1rem;
  text-align: left;
  font-weight: 600;
  color: var(--text-color);
}

table td {
  padding: 0.75rem 1rem;
  border-bottom: 1px solid var(--border-color);
}

table tbody tr {
  transition: background-color 0.2s;
}

table tbody tr:hover {
  background-color: var(--card-hover-bg);
}

table a.problem-title {
  color: var(--text-color);
  font-weight: 500;
  text-decoration: none;
}

table a.problem-title:hover {
  color: var(--link-color);
  text-decoration: underline;
}

/* Theme Toggle */
#theme-toggle {
  background: none;
  border: 1px solid var(--border-color);
  color: var(--text-muted);
  padding: 6px;
  border-radius: 6px;
  cursor: pointer;
  transition: all 0.2s;
  display: flex;
  align-items: center;
  justify-content: center;
}

#theme-toggle:hover {
  background-color: var(--card-hover-bg);
  color: var(--text-color);
}

/* Responsive Design */
@media (max-width: 768px) {
  .hero-section h1 {
    font-size: 2rem;
  }

  .hero-section p {
    font-size: 1.1rem;
  }

  .category-grid {
    grid-template-columns: repeat(auto-fit, minmax(140px, 1fr));
    gap: 1rem;
  }

  .category-card {
    padding: 1.5rem;
  }

  .problem-card {
    flex-direction: column;
    align-items: flex-start;
    gap: 1rem;
  }

  .problem-card>div:last-child {
    align-self: flex-start;
  }

  header .container {
    flex-direction: column;
    gap: 1rem;
  }

  nav a {
    margin: 0 0.75rem;
    font-size: 0.9rem;
  }

  .stats-grid {
    grid-template-columns: 1fr 1fr;
  }

  .platform-section-header {
    flex-direction: column;
    align-items: flex-start;
    gap: 0.5rem;
  }
}