There is a CSS media query for whether a user prefers a Dark-Mode or Light-Mode color scheme. If you're site is able to provide styling for both modes, then you can hook into those preferences like so:
@media (prefers-color-scheme: dark) {
/* dark-mode styles */
/* perhaps changing some custom properties */
}
@media (prefers-color-scheme: light) {
/* light-mode styles */
/* perhaps changing some custom properties */
}
As of this writing,
prefers-color-scheme
is still in a draft state, but it already has decent browser support.