* { box-sizing: border-box; }
body {
  font-family: system-ui, -apple-system, "Segoe UI", sans-serif;
  max-width: 760px;
  margin: 2rem auto;
  padding: 0 1rem;
  color: #1a1a1a;
  background: #fafafa;
  line-height: 1.5;
}
header { border-bottom: 1px solid #ddd; margin-bottom: 1.5rem; padding-bottom: 1rem; }
h1 { margin: 0; }
.tagline { color: #555; margin: 0.25rem 0 0; }
h2 { margin-top: 2rem; }
ol li { margin: 0.4rem 0; }
a { color: #0a58ca; text-decoration: none; }
a:hover { text-decoration: underline; }
.tag {
  display: inline-block;
  font-size: 0.75rem;
  padding: 0.1rem 0.5rem;
  border-radius: 0.5rem;
  margin-left: 0.5rem;
  color: white;
}
.tag.easy   { background: #2e7d32; }
.tag.medium { background: #ed6c02; }
.tag.hard   { background: #c62828; }
code { background: #eee; padding: 0.1rem 0.3rem; border-radius: 3px; font-size: 0.9em; }
#success {
  display: inline-block;
  margin-top: 1rem;
  padding: 0.5rem 1rem;
  background: #2e7d32;
  color: white;
  font-weight: bold;
  border-radius: 4px;
}
.challenge-goal {
  background: #fff3cd;
  border-left: 4px solid #ffc107;
  padding: 0.75rem 1rem;
  margin: 1rem 0;
}
button { padding: 0.5rem 1rem; cursor: pointer; }
nav.back { margin-bottom: 1rem; font-size: 0.9rem; }