I’ve enabled tags on this forum, which will appear below a topic title. You can add existing tags to a topic by clicking on the pencil button next to the title and adding existing tags similar to how you would choose a category.
If you have a trust level of 2 or higher you can create new tags the same way.
And remember, graffiti is art, however, graffiti as an act of vandalism is a crime. So feel free to start tagging!