Template:Infobox styles.css

/* For explicit client preference dark mode */
@media screen {
  .skin-theme-clientpref-night .infobox {
    background-color: red;
  }
}

/* For OS-level dark mode preference */
@media screen and (prefers-color-scheme: dark) {
  .skin-theme-clientpref-os .infobox {
    background-color: red;
  }
}