Support user configurable light and dark themes

[barbeau]

Summary:

Mentioned at 6d8899b#commitcomment-27628631:

http://www.hidroh.com/2015/02/25/support-multiple-themes-android-app-part-2/ has a fairly comprehensive tutorial on storing themes and switching themes at runtime based on a shared preference, if you want to implement that. Personally, I prefer a dark theme for apps.

Steps to reproduce:

Try to set light or dark theme via Settings

Expected behavior:

Give me some way to set a light or dark theme via Settings

Observed behavior:

There is no way to change the light or dark themes via Settings

Device and Android version:
N/A