This is a minimum viable implementation for using Matplotlb style sheets. Currently light and dark style sheets are hardcoded (taken from#157), instead of being generated from the napari theme. I think this is a better approach, as even though it's less general, I doubt the napari folks will be changing their theme very often so manually updating our theme when they do is a reasonable ask.

This doesn't deal with whether we should look for user style sheets, but does let the users set the style sheet property with a path to their own style sheet. We can follow this PR up with a PR to add a location where users can put a custom style sheet.

This still needs:

• A test with a custom style sheet
• A (manual?) check on how this works with theme changing - this works as well as before, but still subject toTheme change doesn't work perfectly whilst widget is open. #140