Yeah, I was wondering if that would be helpful too, but I think this person wants to decorate other topic elements based on the existence of a tag, and that will be a tad more difficult, if not impossible to do in CSS alone.
There are a couple of things that could be done in Core to make this easier, such as, applying the tag classes to the topic entity itself on the topic listing and the actual topic.
Beyond that, I believe JavaScript would have to be used to read the tag class inside an entity and then apply a new class to its parent. But thoes are my thoughts on it as of right now.