Add colour scheme switch and black & white theme

[mdusher]

Hello,

I added a colour scheme switcher to the Preferences page and added a second colour scheme.

I've called the second colour scheme "Black White" because the light theme is mostly Black text on a white background and the dark theme is mostly white text on a black background (creative, I know).

I added it mostly because it suits my own preference but thought I'd do a pull request in case you'd like to include it (this is my first time touching any sort of Android dev so happy to modify anything if you have a preferred way of doing things).