Thought I'd give everyone a quick tutorial on how to make different username styles per different themes. If you ever had let's say a light theme and a dark, you could fix your usergroups for each one so they don't conflict and look like shit.
Step 1 - Go to your ACP > Users & Groups > Group you wish to change
Step 2 - Find your group's username style. So for the admin for example let's just say it looked like this: