Unfortunately there's no way to change usergroup colors on a per-style basis. Both colors are the same for both styles. It's really designed for the light style while also being readable on the dark style. Which isn't as bad as it looks on the dark style.
There's honestly no way you could make it much better. You don't like it because it seems bright to you on the dark style but if I dulled the colors down and made them more placid it would have the opposite effect and would look faded on the black. I may tinker with it later and possibly see if I can make minor tweaks so it isn't as bright but they would need to stay minor or it would look faded. Being bright is actually better then not being readable.
Speaking of retired staff/vip there is a chance I may possibly change VIP to purple later but this is only a small possibility. If I did it would be a ways off. In the next couple months I may tone down the brightness a tad bit on the moderator colors. I will keep you posted. Thank you for your inquiry.
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