Problem

The icons are hard-coded to be black and blend into the backgound:

Proposed solution

For wx and Qt, we modify the color to be light when a dark backgound is present; see the _icon() methods in these backends.

It would be nice to have something similar for Tk as well.

This needs somebody with knowledge in Tk. Marking as good first issue because one does not need to understand anything about Matplotlib. The button icon is set in NavigationToolbar2Tk._set_image_for_button().