Default Theme - use prefers-color-scheme to detect system theme

Title is fairly descriptive - now that themes are an option, it would be nice to match default Pi-hole theme to user's system theme leveraging the prefers-color-scheme CSS feature: https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-color-scheme

Cheers, and thanks for the great update!

Implemented a long time ago:
https://github.com/pi-hole/AdminLTE/pull/1836
https://github.com/pi-hole/AdminLTE/pull/2006