Refactor the CSS rules for the overlayContainer

[Snuffleupagus]

This PR refactors the CSS for the overlayContainer, to achieve the following goals

• Reduce unnecessary duplication of CSS rules.
• Remove the dependency on doorHanger, since using that class seems out of place here.
• Serve as a prerequisite to a follow-up PR I'm working on, that will change how we interact with the overlay, since the current way of doing this won't scale well if we want to add more overlays (e.g. UI for viewing and setting default preferences #3985).

/cc @timvandermeij Would you mind reviewing this?

[timvandermeij]

If I am not mistaken, this could also help to address #3783 more easily. I will certainly review this.

[timvandermeij]

@Snuffleupagus This looks perfectly fine to me once the ID's have been removed. Nice work (and you also thought about the RTL layout which I usually tend to forget)!

[Snuffleupagus]

The PR is updated to address the review comments. Note that I didn't think too much about the re-usability of the CSS, I only wanted to do some clean-up for an upcoming patch.

/botio-linux preview

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[timvandermeij]

Thanks!