Add dark-mode

[Florian Schlittenbauer]

As this is for fancy index styling it should absolutely have a fancy darkmode. I suggest using the prefers-color-scheme media-query (see https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-color-scheme) like so:

@media (prefers-color-scheme: dark) {
/* do the dark stuff */
}

Maybe I'll find some time the next days to do a proper PR...