Improve compatibility with dark color schemes

I've discovered a big CSS problem in my add-on and checked some other add-ons, to see you are also affected.

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

That includes add-on options page (presumably only if they use browserStyle?).

This is how your add-on looks like then:

See https://github.com/rugk/offline-qr-code/issues/190 for details.

Thanks to the grey style, it does not look that bad, but I assume you anyway want a lighter color for the dark mode.

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I've asked some questions about that change at https://discourse.mozilla.org/t/firefox-68-added-dark-version-to-about-addons-breaking-many-add-ons/38716?u=rugkx to clarify when/how this will be in stable.