As far as I know, the Unified Extensions Button is the first themed panel to make prominent use of icons. Unfortunately, many extensions fail to provide icons that will have ideal contrast in both light and dark themes. To remedy this problem, I propose that we add something like a dark_icons key to manifest.json

Extensions with a custom button can already specify theme_icons (introduced in bug 1329242).
There is not a way to specify an icon for dark themes, but a standardized way to do that would be welcome.

FWIW, there is a discussion on customizing the icons for dark themes at https://github.com/w3c/webextensions/issues/229 .
The discussion seems to have been narrowed towards extension action button icons instead of the extension icon itself, but I wouldn't mind revisiting the discussion to cover extension icons in general.

We are going to fix this bug, see also: https://github.com/w3c/webextensions/issues/229#issuecomment-1418903935