already breaks steam TOS, and literally anyone who wants a different colored avatar border is doing it for exclusivity, especially with the gold ones (although some people just tried it to see if it worked).
the mod is too new to see what steam will say about it. they'll either say that it's okay or that "people who did this broke the ToS and they're getting bans." colored usernames are cool and all, but they have so little of a use that it's barely worth changing (imagine if you could have username colors on the blf - few people would care).