Subtheme functions

[teunbrand]

This PR implements the subtheme idea from #5301.

Briefly, it adds functions that allow specifying parts of a theme with shorter arguments.

A few considerations:

• I tried making a function factory for these subtheme functions, to automatically populate the formals as the argument names are very regex friendly. However, roxygen2 wouldn't recognise the formals in the @usage section.
• Should these function be named subtheme_*() as to avoid confusion with e.g. theme_light() which is conceptually doing a different thing?