Add colour scheme switcher with Black & White and Material You 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).

[Lokito23]

On the mentioned issue, I managed to add Material You support to this PR, though I'm unsure if you would like if I commited this functionality into the PR.

It still requires a method to check if the Android version is too old, in order to disable the option, but besides that it seems like it works.

[mdusher]

I've pulled in the 'develop' branch and updated the PR to match the more recent changes.

@Lokito23 I'm happy to pull your changes into this PR if you have them available somewhere

[mdusher]

Adding that this would likely address #120

[Lokito23]

Couldn't manage to rebase, so I just redid my changes, currently trying if it still works.

My guess is that to contribute to the PR, I have to open a PR on the PR, correct me if I'm wrong.

Edit: I'll do it that way anyways, also it does work.

[mdusher]

Thanks, I gave your changes a quick test and I've merged your pull request into my branch (and updated the title of this PR to match)