Update dark theme: Fix notifications, popover and progress bar. Update colors.

Hi!

I am not a CSS expert so if you find mistakes, please, help me to fix them!

I had to use !important in most parts of the code to make it work on my local installation but if you think it should not be necessary, just tell me!

Code can be a bit hacky sometimes but it should be good enough until main PeerTube css get a cleanup (I think it's on the new roadmap for this year). Once achieved, we'll need to rework all themes anyway.