Add to common problems pitfall with dynamicaly selected themes

[jksmilton]

Not sure if it is a common issue per se, but its one that tripped me up for a good while: we're applying themes dynamically in a way that means tailwind ends up stripping out the themes from the css. The fix is to add your possible theme selectors to the tailwind safelist so that tailwind doesn't helpfully strip them from the final css.

Maybe should go under a header like: "The generated CSS is missing the defined themes" or could be added to the "generated css is missing classes and variables" (this is the same issue effectively, but the current wording is different enough from what I was seeing that it didn't help as we weren't just missing some classes but a whole theme).