* {
  box-sizing: border-box;
}

body {
  margin: 0;
  background: #071321;
  color: #eef6ff;
  font-family: Arial, Helvetica, sans-serif;
}

.lesson {
  max-width: 1100px;
  margin: 18px auto;
  padding: 20px;
  background: #10233a;
  border: 1px solid #294a67;
  border-radius: 18px;
}

h1 {
  text-align: center;
  margin: 6px 0 10px;
  font-size: clamp(22px, 3vw, 31px);
}

.subtitle {
  text-align: center;
  color: #bdd5e6;
}

.stage {
  overflow: hidden;
  border: 1px solid #35536c;
  border-radius: 14px;
  background: #03070c;
}

svg {
  display: block;
  width: 100%;
  height: auto;
}

.beam-label {
  font: 600 34px Arial, sans-serif;
  fill: #ffffff;
  paint-order: stroke;
  stroke: #071321;
  stroke-width: 4px;
}

.controls {
  display: flex;
  gap: 12px;
  flex-wrap: wrap;
  margin: 18px 0;
}

button {
  border: 0;
  border-radius: 9px;
  padding: 12px 20px;
  font-size: 15px;
  font-weight: bold;
  cursor: pointer;
  background: #1db9cc;
  color: #071321;
}

#replay {
  background: #e6b85d;
}

#reset {
  background: #304a63;
  color: white;
}

.explanation {
  background: #172e47;
  border-left: 4px solid #1db9cc;
  border-radius: 8px;
  padding: 16px;
  line-height: 1.6;
}

.explanation h2 {
  margin: 0 0 8px;
  font-size: 21px;
}

.explanation p {
  margin: 8px 0;
}

@media (max-width: 600px) {
  .lesson {
    margin: 0;
    padding: 10px;
    border-radius: 0;
  }

  .controls button {
    flex: 1;
  }
}