Correct icon theme should be automatically selected in system variant

[spymastermatt]

Thunderbird uses the [brightext] attribute on certain elements to determine if a light or dark text-color is required for toolbars etc.

We should try to use that to update other elements and select the correct icon theme automagically

[spymastermatt]

I did look into this but since the places where the icons are needed are not children or siblings of elements with the brighttext attribute, it seems difficult to use this. Further, :-moz-lwtheme-brighttext only works if the theme creator has specified whether bright text should be used