I guess we could add classes to the code here and there if it helps with custom css. Perhaps we could build a list of what elements should have a class to get a sense of what's needed.
Long term I'd like to switch to scss for styling, so nearly everything will have classes then or be selectable with css. At the moment it's rather tedious to create UI in Joplin because you need to manually set the styling on each tag.
Switching to scss would allow proper styling, and we can use the dynamic properties of scss to support themes.