Feature: Dark mode and user styles

[daniel-simpson]

Initial implementation for comment.

Addresses dark mode request: #57 along with my comment for a place to inject user-defined styles.

I'm expecting that the colors I've picked for dark mode (especially the accent colors) may need some feedback, but it's a starting point.

Thanks again for your fantastic frontend project!

[daniel-simpson]

A few example screenshots:

[karlomikus]

I already started some small work on theme switching. It's a larger change so it will take some time to implement. This looks like a good starting point, thanks!

[karlomikus]

I'm closing this in favor of current develop branch changes.