Add a feature to allow skinning

[markhowellsmead]

In the 1990s, we were able to select from a range of Windows “skins” to make our computers look how we wanted them to. The underlying OS interface didn't change, but we were able to personalise the colour scheme to match our own taste.

With the move to ensure that Themes are function-agnostic—plugins for functionality!—the last two Core Themes seem to show that by providing a colour scheme-agnostic set of patterns and content layouts, it would be much more feasible to provide the option to “skin” the site by selecting an alternative theme.json. This mechanism is already in place for individual themes, where it's possible to choose Style Variations.

By adding the possibility to select predefined Style Variations from a central repository, users could change the look and feel of their entire sites without having to go through (potentially) thousands of posts and pages and rebuild them all to work with an updated look and feel. This would, naturally, require that the naming convention for fonts, colours and spacing units be standardised. But this should be comparatively simple to ensure in upcoming standard themes.

If this feature were to be made available and was heavily pushed and prominently documented, this would encourage theme developers to adhere to standardised naming conventions.

[markhowellsmead]

See #45371.