We’ll need to add either a title or href attribute for this to work. Unlinke tags in preview, tags in the tag pane don’t have a href yet.
I thought :first-letter would work, but unfortunately it doesn’t. # is not considered to be a letter by CSS spec.
So for both of these to work, we’ll need to make changes on our end.
Are you trying to make a new theme, if I may ask?