[Feature] Add dark mode

[Teraskull]

Currently, the main option is to use the Dark Reader extension.

It works fine, but some elements are too contrasting, and there is FOUC on reload, when some elements are shown in light theme for a second.

To use it, you have to install the extension, which some users don't want to, or can't. Also, you can't have extensions on mobile.

Implementing built-in dark mode solves all above issues.