Implement a native looking theme again

[Croydon]

The former "none" theme, which used Firefox's UI CSS got removed, since this wasn't possible anymore with a WebExtension due to sandboxing.

Starting with Firefox 58 it will be possible again.

https://bugzilla.mozilla.org/show_bug.cgi?id=1349944
https://bugzilla.mozilla.org/show_bug.cgi?id=1415872
https://bugzilla.mozilla.org/show_bug.cgi?id=1347188

I plan to reimplement it at some point. It might be necessary to wait a bit longer even after the release of Firefox 58 till the point, when Firefox themes are supporting more attributes again.

[nt1m]

browser.theme.getCurrent() only works with webextension themes (like VivaldiFox or Container theme). For the default theme, it will just return {}.

[ntninja]

@nt1m: But you should be able to use the legacy system colors (contain values from the current system's theme), when no overrides by the browser theme are set.

[Croydon]

I'm likely going to postpone this till the current very active work on themes is finished. Many new theme attributes are getting added and the default themes are getting refactored, both progresses may be of good use.