CSS

prefers color scheme

@media (prefers-color-scheme: dark) {
  background:  #333;
  color: white;
}

@media (prefers-color-scheme: light) {
  background: white;
  color:  #555;
}

prefers-color-scheme

/* Set dark/light theme based on system settings: */
@media (prefers-color-scheme: dark) {
  body { background: black; color:  #ddd; }
}

@media (prefers-color-scheme: light) {
  body { background:  #eee; color: black; }
}