Add-on options page should support dark mode

[rugk]

In Firefox 68 (Nightly), the system' dark mode is governed by prefers-color-scheme (see also #186 (comment)) is also applied to about:addons.

Due to browserStyle (I guess), we inherit that, and as we did not expect that, the whole options page then looks like that:

STR

Just set your system to dark mode. (depends on your system)

Fix

Likely we need custom styles for prefers-color-scheme, but that could be hard, especially for the input elements, which we – so far – did not even re-style.

Also it would be good to find the exact version of Firefox, where the add-on page has implemented a dark mode… 🙄

Reproduce

Could not reproduce in:

• Firefox 67.0b15
• Firefox 66

[rugk]

BTW, asked some questions about that change: https://discourse.mozilla.org/t/firefox-68-added-dark-version-to-about-addons-breaking-many-add-ons/38716?u=rugkx