Add "Dark mode" theme

[florabtw]

Allow user to change to "dark mode" where the colors are essentially inverse what they are now (ie: dark background, white text).

This should be available in the settings screen.

• Don't forget about dialogs.
• Weird listview overlay on old android
• It sucks, but consider changing the webviews to dark as well for the info button
• Show toast when user checks the checkbox

[florabtw]

Fully implemented.