Implement a new theme system #174
Description
Branch: feature/textual-overhaul
Add a footing binding that opens a new page/interactive popup. When the user exits this UI screen, it should send them back to where they were before entering it.
In this new UI space, there should be selectors for UI presets with clear descriptions/examples of what the UI looks like. If you can include a small rendering window with example text, so the user can see the font + colors, that is acceptable. Add the description to the side of this window.
You do not need to migrate the old schemes; they will be deprecated.
As an example of Textual's utility, create a couple of initial themes with different colors and fonts.