Implement light and dark theme by defautl - eviltoast

A while back I made a light and a dark theme for lemmygrad.ml. Unfortunately, due to current code restrictions, a custom selected instance theme doesn’t have a light and dark mode.

As suggested by Nutomic, I would like to request that the new themes replace the litely and darkly files so that the red theme can default according to browser preferences. I recognize that this may be hacky and might need to be reapplied on code updates, but it seems like the best solutions. If needed (I don’t think it is), the original litely and darkly themes can be renamed to litely-green and darkly-green to keep them available.

(@muad_dibber@lemmygrad.ml )