Colour themes (can be set by user)

[ewels]

Following on from #212 to keep a narrow-scope issue:

It'd be have the ability to use and create themes in rich-click. Possibly to be edited by tool developers, but mostly to ship with the library itself and / or be created by end users.

This ties into accessibility because an end user could then set RICH_CLICK_THEME=deuteranopia (or whatever) and have an appropriate colour theme applied across all tools using rich-click.

For bonus points, we should make this easy to set at user level to configure tools to use styles that they like best.

This will likely best be implemented by using Styles within rich-click, rather than directly referencing colours within the code.