Show better icon in dark theme

[zieren]

The FF dark theme modifies the icon's colors. Unfortunately black becomes white, but white remains white. I don't know if that is simply a bug or caused by the specific icon; I don't know any other page action extensions.

This means that the icon is just a white blob and the on/off versions are hard to tell apart. It looks like this is a common problem.

[zieren]

It might make sense to change the extension from a page_action to a browser_action. That's technically incorrect because it is page-specific, but page_action seems to be poorly supported (this issue being one case in point; another one being usability).