Minor visual changes. Added support for dark mode.

The toolbar button color now updates along with the theme you're using.

Right now, it only works for themes you've installed yourself, because Firefox doesn't let developers know when you've switched to the bundled Light or Dark themes (https://bugzilla.mozilla.org/show_bug.cgi?id=1386004).
Specifically, the button uses the color from either the toolbar icon or text.