Extracted CSS variables to css-variables.css to allow user GUI style customization

[JWahle]

CamillaGUI is now styleable. Here is what I did:

• extracted essential style properties (mainly colors & borders) into CSS variables in public/css-variables.css
  • this includes a new dark skin (similar to moOde)
  • and the old black/white style (has to be manually enabled). If it gets too cumbersome to maintain both, we can drop one anytime. We might also add a switch to have the user change it at runtime.

I also have a different implementation in the branch gui-customization that loads the style from the backend's config dir, but I like this one better, since it's simpler and does not require commits to the frontend + backend repos for style changes.

@HEnquist what do you think of the new style? Should this become the new default, or should we keep the old one?

[JWahle]

@bitkeeper I already have a moode-style skin ready for you to try, once this is merged.

[HEnquist]

I think this looks good, let's make it the default. I played around and I would modify the defaults a little. The borders are very thin and bright, and I think it looks better if they are a little wider and darker.

Also the pipeline plot looks a little dull now. I think the solid backgrounds of the devices and mixers were helpful. I did a quick test and added a separate bg color for the frames and boxes, and a text color for the boxes:

[JWahle]

Looks better. I pushed my changes to the gui-style branch on this repo, so you can add your changes there. This PR would then be obsolete.

[JWahle]

Closed in favor of #36