Rendering issues caused by latest Nightly upates

[Daktyl198]

A change recently landed in Nightly to make hanger popups use a dark/black background when using the default Dark Firefox theme. For example, the Library popup now looks like this:

This makes it so that the background of the popup behind the feed list in Feedly Notifier is black while the feed list itself has a white background. Very jarring.

(feed list censored)

There are other addons also affected by this issue, e.g. Open With or Search By Image.