* {
  box-sizing: border-box;
  font-family: system-ui, sans-serif;
}

body {
  background: #0f172a;
  color: #e5e7eb;
  margin: 0;
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}

.top-nav {
  display: flex;
  justify-content: center;
  padding: 1rem 1.5rem 0.5rem;
}

.back-link {
  display: inline-flex;
  align-items: center;
  gap: 0.4rem;
  padding: 0.55rem 1rem;
  border-radius: 999px;
  border: 1px solid #1f2937;
  background: #0b1222;
  color: #e5e7eb;
  text-decoration: none;
  font-weight: 700;
  transition: transform 0.12s ease, background-color 0.12s ease, border-color 0.12s ease;
}

.back-link:hover {
  background: #111827;
  border-color: #38bdf8;
  transform: translateY(-1px);
}

.back-link:active {
  transform: translateY(0);
}

header {
  text-align: center;
  padding: 2rem;
  background: #020617;
}

main {
  flex: 1;
  display: grid;
  place-items: center;
}

#lesson {
  background: #020617;
  padding: 2rem;
  border-radius: 12px;
  width: 320px;
  text-align: center;
  box-shadow: 0 10px 40px rgba(0,0,0,.5);
}

#question {
  font-size: 4rem;
  margin: 1rem 0;
}

input {
  width: 100%;
  padding: .6rem;
  font-size: 1rem;
  border-radius: 6px;
  border: none;
  margin-bottom: .8rem;
}

button {
  width: 100%;
  padding: .6rem;
  font-size: 1rem;
  background: #38bdf8;
  border: none;
  border-radius: 6px;
  cursor: pointer;
}

button:hover {
  background: #0ea5e9;
}

#feedback {
  margin-top: 1rem;
  font-weight: bold;
}

#stats {
  margin-top: 1rem;
  display: flex;
  gap: 1.5rem;
  justify-content: center;
}

footer {
  text-align: center;
  padding: 1rem;
  font-size: .8rem;
  opacity: .6;
}