#sskl-app {
  font-family: Arial, sans-serif;
  color: #fff;
  background: url('https://upload.wikimedia.org/wikipedia/commons/5/5f/Milky_Way_Arch.jpg') no-repeat center center fixed;
  background-size: cover;
  padding: 20px;
}
#sskl-app h1 {
  text-align: center;
}
#sskl-app .solar-system {
  position: relative;
  height: 600px;
}
#sskl-app .sun {
  position: absolute;
  top: 50%;
  left: 50%;
  width: 80px;
  height: 80px;
  background: radial-gradient(circle, yellow, orange, red);
  border-radius: 50%;
  transform: translate(-50%, -50%);
  z-index: 10;
}
#sskl-app .orbit {
  position: absolute;
  top: 50%;
  left: 50%;
  border: 1px dotted rgba(255,255,255,0.3);
  border-radius: 50%;
  transform: translate(-50%, -50%);
  animation: spin 40s linear infinite;
}
#sskl-app .planet {
  position: absolute;
  top: 50%;
  left: 100%;
  transform: translate(-50%, -50%);
  border-radius: 50%;
  cursor: pointer;
}
#sskl-app .planet:hover {
  animation: rotate 1s linear infinite;
}
@keyframes rotate {
  from { transform: translate(-50%, -50%) rotate(0deg); }
  to { transform: translate(-50%, -50%) rotate(360deg); }
}
@keyframes spin {
  from { transform: translate(-50%, -50%) rotate(0deg); }
  to { transform: translate(-50%, -50%) rotate(360deg); }
}
#sskl-app .planet-list button {
  display: block;
  margin-bottom: 10px;
  padding: 5px 10px;
  background: #444;
  border: none;
  border-radius: 5px;
  color: #fff;
  cursor: pointer;
}
#sskl-app .info-section,
#sskl-app .quiz-section {
  background: rgba(0,0,0,0.6);
  padding: 20px;
  margin-top: 30px;
  border-radius: 10px;
}
#sskl-app details {
  margin: 10px 0;
  background: #111;
  padding: 10px;
  border-radius: 5px;
}
#sskl-app summary {
  font-weight: bold;
  cursor: pointer;
}