I used the Development Tools built into Joplin. I then highlighted some editor text and "probed it" looking for a suitable class. They are not always easy to find.
By the way, when trying to tweak parts of Joplin you may find a class that seems to control the bit you want to change but it has a weird name like UVbgHt or LkYmvd. Resist the temptation to use these as they are machine generated and will most likely change when a new version is built.