/* === VARIABLES === */
:root {
  --bg-color: #f4f7f9;
  --text-color: #1a202c;
  --card-bg: #ffffff;
  --border-color: #e2e8f0;
  --input-bg: #ffffff;
  --accent-color: #3182ce;
  --accent-color-hover: #2b6cb0;

  /* Badge Colors */
  --easy-bg: #d4edda;
  --easy-text: #155724;
  --medium-bg: #fff3cd;
  --medium-text: #5a4501;
  --hard-bg: #f8d7da;
  --hard-text: #721c24;
}

[data-theme="dark"] {
  --bg-color: #1a202c;
  --text-color: #e2e8f0;
  --card-bg: #2d3748;
  --border-color: #4a5568;
  --input-bg: #2d3748;
  --accent-color: #63b3ed;
  --accent-color-hover: #90cdf4;

  --easy-bg: #1c4532;
  --easy-text: #68d391;
  --medium-bg: #4a3a19;
  --medium-text: #f6e05e;
  --hard-bg: #521b22;
  --hard-text: #f56565;
}

/* === GLOBAL RESET & BASE === */
*, *::before, *::after {
  box-sizing: border-box;
}

body {
  margin: 0;
  font-family: 'Inter', sans-serif;
  background-color: var(--bg-color);
  color: var(--text-color);
  line-height: 1.5;
  transition: background-color 0.3s ease, color 0.3s ease;
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}

.container {
  max-width: 1200px;
  margin: 0 auto;
  padding: 0 1.5rem 2rem;
  width: 100%;
}

/* === HEADER === */
.header {
  background: var(--card-bg);
  border-bottom: 1px solid var(--border-color);
  box-shadow: 0 4px 6px rgba(0,0,0,0.05);
  position: sticky;
  top: 0;
  z-index: 100;
  padding-top: 20px;
  flex-shrink: 0;
}

.header__container {
  display: flex;
  justify-content: space-between;
  align-items: center;
  height: 70px;
}

.header__logo .logo-text {
  color: var(--accent-color);
  font-size: 1.8rem;
  font-weight: 700;
  letter-spacing: 0.5px;
}

.header__nav {
  display: flex;
}

.nav__list {
  list-style: none;
  display: flex;
  gap: 2rem;
  margin: 0;
  padding: 0;
  transition: all 0.3s ease;
}

/* Mobile nav (hidden by default) */
.nav__list.open {
  display: flex !important;
}

.nav__link {
  color: var(--text-color);
  text-decoration: none;
  font-weight: 500;
  font-size: 1rem;
  position: relative;
  padding: 0.25rem 0;
  transition: color 0.3s ease;
}

.nav__link.active,
.nav__link:hover,
.nav__link:focus {
  color: var(--accent-color);
}

.nav__link.active::after,
.nav__link:hover::after,
.nav__link:focus::after {
  content: '';
  display: block;
  width: 100%;
  height: 2px;
  background-color: var(--accent-color);
  position: absolute;
  bottom: -4px;
  left: 0;
  border-radius: 2px;
}

/* Hamburger toggle */
.header__toggle {
  display: none;
  flex-direction: column;
  gap: 6px;
  width: 28px;
  height: 22px;
  background: none;
  border: none;
  cursor: pointer;
}

.hamburger {
  width: 100%;
  height: 3px;
  background-color: var(--text-color);
  border-radius: 2px;
  transition: background-color 0.3s;
}

/* === HERO === */
.challenges-hero {
  background: linear-gradient(135deg, var(--accent-color), #94bbe9);
  padding: 5rem 0 3rem;
  text-align: center;
  color: white;
}

.challenges-hero__title {
  font-size: 3.5rem;
  font-weight: 700;
  margin-bottom: 1rem;
}

.challenges-hero__description {
  font-size: 1.25rem;
  max-width: 660px;
  margin: 0 auto;
  opacity: 0.9;
}

/* === FILTER CONTROLS === */
.challenges-filter {
  background-color: var(--card-bg);
  padding: 1.5rem 0;
  border-bottom: 1px solid var(--border-color);
}

.filter-controls {
  display: flex;
  flex-wrap: wrap;
  gap: 1.5rem 2rem;
  justify-content: center;
  align-items: center;
}

.filter-group {
  display: flex;
  flex-direction: column;
  min-width: 150px;
}

.filter-group label {
  font-weight: 600;
  margin-bottom: 0.3rem;
  color: var(--text-color);
}

.filter-select,
.search-input {
  width: 100%;
  padding: 0.5rem 1rem;
  border: 1px solid var(--border-color);
  border-radius: 0.5rem;
  background-color: var(--input-bg);
  color: var(--text-color);
  font-size: 1rem;
  transition: border-color 0.3s ease;
}

.filter-select:focus,
.search-input:focus {
  outline: none;
  border-color: var(--accent-color);
  box-shadow: 0 0 6px var(--accent-color-hover);
}

/* === CHALLENGES GRID === */
.challenges-grid {
  padding: 3rem 0 5rem;
}

.challenges-list {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(320px, 1fr));
  gap: 2rem;
}

.challenge-card {
  background-color: var(--card-bg);
  border-radius: 12px;
  padding: 1.5rem;
  border: 1px solid var(--border-color);
  box-shadow: 0 4px 6px rgba(0,0,0,0.05);
  display: flex;
  flex-direction: column;
  transition: box-shadow 0.3s ease, transform 0.2s ease;
  cursor: pointer;
}

.challenge-card:hover,
.challenge-card:focus-within {
  box-shadow: 0 8px 24px rgba(37, 99, 235, 0.3);
  transform: translateY(-3px);
  outline: none;
}

.challenge-card__header {
  display: flex;
  justify-content: space-between;
  align-items: flex-start;
  margin-bottom: 0.75rem;
}

.challenge-card__title {
  font-size: 1.25rem;
  font-weight: 700;
  margin: 0;
  flex-grow: 1;
  color: var(--accent-color);
}

.difficulty-badge {
  padding: 0.25rem 0.75rem;
  border-radius: 9999px;
  font-size: 0.75rem;
  font-weight: 700;
  text-transform: uppercase;
  white-space: nowrap;
  user-select: none;
}

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

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

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

.challenge-card__description {
  flex-grow: 1;
  font-size: 1rem;
  line-height: 1.4;
  color: #555;
  margin-bottom: 1rem;
}

.challenge-card__meta {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
  margin-bottom: 1.25rem;
}

.category-tag {
  background-color: var(--accent-color);
  color: white;
  padding: 0.2rem 0.6rem;
  border-radius: 9999px;
  font-size: 0.75rem;
  text-transform: capitalize;
  user-select: none;
}

.challenge-card__actions {
  text-align: right;
}

.btn {
  border: none;
  border-radius: 0.5rem;
  padding: 0.55rem 1.1rem;
  font-size: 0.9rem;
  font-weight: 700;
  cursor: pointer;
  text-decoration: none;
  display: inline-block;
  transition: background-color 0.3s ease;
  user-select: none;
}

.btn-sm {
  font-size: 0.85rem;
  padding: 0.4rem 0.8rem;
}

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

.btn-primary:hover,
.btn-primary:focus {
  background-color: var(--accent-color-hover);
  outline: none;
}

/* No results message */
.no-results-message {
  text-align: center;
  color: #718096;
  padding: 3rem;
  font-size: 1.25rem;
}

/* === FOOTER === */
.footer {
  background-color: var(--card-bg);
  padding: 2rem 1.5rem;
  border-top: 1px solid var(--border-color);
  color: var(--text-color);
  font-size: 0.9rem;
  flex-shrink: 0;
}

.footer__content {
  display: flex;
  justify-content: space-around;
  flex-wrap: wrap;
  gap: 2rem;
  margin-bottom: 1.5rem;
}

.footer__section h4 {
  font-weight: 700;
  margin-bottom: 0.8rem;
  color: var(--accent-color);
}

.footer__section ul {
  list-style: none;
  padding: 0;
  margin: 0;
}

.footer__section a {
  color: var(--text-color);
  text-decoration: none;
  font-weight: 500;
  transition: color 0.3s;
}

.footer__section a:hover,
.footer__section a:focus {
  color: var(--accent-color);
  outline: none;
}

.footer__bottom {
  text-align: center;
  color: #94a3b8;
  font-size: 0.85rem;
}

/* === THEME TOGGLE BUTTON === */
#theme-toggle {
  position: fixed;
  right: 1.5rem;
  bottom: 1.5rem;
  background: var(--card-bg);
  border: 1px solid var(--border-color);
  border-radius: 50%;
  padding: 0.6rem;
  cursor: pointer;
  box-shadow: 0 2px 8px rgba(0,0,0,0.1);
  transition: background-color 0.3s ease;
  z-index: 1001;
}

#theme-toggle svg {
  display: block;
  width: 24px;
  height: 24px;
  stroke: var(--text-color);
}

/* === RESPONSIVE === */
@media (max-width: 768px) {
  .nav__list {
    position: absolute;
    top: 70px;
    right: 0;
    background-color: var(--card-bg);
    width: 220px;
    flex-direction: column;
    padding: 1rem;
    border-radius: 0 0 8px 8px;
    box-shadow: 0 4px 12px rgba(0,0,0,0.15);
    opacity: 0;
    visibility: hidden;
    transform: translateY(-10px);
    transition: opacity 0.25s ease, visibility 0.25s, transform 0.25s ease;
    z-index: 1000;
    display: none; /* hide by default */
  }

  .nav__list.open {
    opacity: 1;
    visibility: visible;
    transform: translateY(0);
    display: flex; /* show when open */
  }

  .nav__link {
    padding: 0.7rem 1rem;
    font-size: 1rem;
  }

  .header__toggle {
    display: flex;
    flex-direction: column;
    gap: 6px;
    width: 28px;
    height: 22px;
    background: none;
    border: none;
    cursor: pointer;
  }

  .hamburger {
    width: 100%;
    height: 3px;
    background-color: var(--text-color);
    border-radius: 2px;
    transition: background-color 0.3s;
  }

  .filter-controls {
    flex-direction: column;
    gap: 1rem;
  }

  .search-group {
    margin-left: 0;
    width: 100%;
  }

  .search-input {
    width: 100%;
  }

  .challenges-list {
    grid-template-columns: 1fr;
  }

  .challenges-hero__title {
    font-size: 2.5rem;
  }
}

@media (max-width: 480px) {
  .container {
    padding: 0 1rem 2rem;
  }

  .challenges-hero__title {
    font-size: 2rem;
  }
}