@lislegaard @eaterofsnacks
There are different ways of doing it, but the most common and compatible way is with a media query. It’s really a matter of defining the colors for light and dark. Something like :

:root {
--bg: #fff;
--text: #000;
}
@media (prefers-color-scheme: dark) {
:root {
--bg: #000;
--text: #fff;
/* filter: brightness(.8) contrast(1.2); */
}
}