Explore adding mobile options to themes/modules

[jasonnrosa]

Per the discussion here we should explore the idea of having mobile options for fields (in particular font sizes). This is something typically added by developers to solve mobile responsiveness issues (e.g. if you have a font size of 48px on desktop you likely don't want the same for mobile) so we should figure out the best way to approach this and ideally have the starting point be fairly simple so developers can build off of it to meet their specific use case.

CC: @johnsfuller @JacobLett