How can I whitelist a class in a theme?

[sam (Sam Saffron)]

In general we do blocks in bbcode, so it gets a bit confusing to add another pattern.

The issue here is the whitelister per:

https://github.com/discourse/discourse/blob/master/app/assets/javascripts/pretty-text/white-lister.js.es6#L114-L179

But our DSL is a bit too limited atm to add theme-* as a whitelisted class so we need to fix that.