Added support for custom themes

[winston-de]

Closes #3809
This PR adds base support for theming, and selecting themes.
Not to be confused with light/dark themes, as these themes can utilize both.

You can test this by adding themes to the themes folder that should be created when you run the branch.

Themes are in the form of xaml files with resource dictionaries.

Basic example of theming:

[winston-de]

@yaichenbaum x:load doesn't seem to support Binding, and x:Bind doesn't seem to work with the datacontext. To get around this, I moved ShowColorSchemeSelector to code-behind. Is this ok?

[yaira2]

@yaichenbaum x:load doesn't seem to support Binding, and x:Bind doesn't seem to work with the datacontext. To get around this, I moved ShowColorSchemeSelector to code-behind. Is this ok?

@winston-de That should be fine in this case.

[yaira2]

@winston-de Nice work! LGTM

[d2dyno1]

@winston-de How does a theme file look like? How do I create one myself?

[yaira2]

@winston-de How does a theme file look like? How do I create one myself?

You can find instructions here https://files-community.github.io/docs/#/articles/custom-themes.