Enable fine-grained styling in code

[msujew]

What it does

Closes #11063
Closes #6743

Adds a new StylingService (and an associated StylingParticipant) which allows us to dynamically change styles using code. This has the advantage that we can set styles based on the existence of a color in a color theme.

As a simple example, we can take the notification hover color, which isn't set for some themes (hc-black). Hovering over a notification when the color isn't set will make the background transparent, while it should usually just keep the existing background color.

I used this feature to also implement the missing capabilities for high contrast theming and clean up some more (unused) css files (such as the variables-dark.useable.css/ variables-bright.useable.css files).

How to test

1. Open VSCode and Theia side by side
2. Select the dark high contrast theme for both of them
3. Assert that they behave (close to) the same with the high contrast theme
4. Assert that no visual regressions appear on other themes

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[colin-grant-work]

Looks super good 👍, and you've made it look easy! I thought it would take a lot more code than this to implement the feature. I haven't noticed any regressions in a quick look, but I'll leave an HC theme in place for a bit to see if anything comes up. A few comments on the code.

[msujew]

@colin-grant-work Thanks, I split the styling participants related to a certain area of the app into one class each 👍

[colin-grant-work]

With the modularization, it should be easier for downstream applications to customize or add to particular parts. 👍