BuddyPress.org

I think you would have to go into buddypress it’s self and change the css.

I was able to do a fair amount of style customization to BP but I’m using custom theme designed to work with bp so not sure how others would go about it.

https://buddypress.org/community/groups/buddypress-group-css/

That being said you might want to give this plugin a look and see how it does things and then adapt and aplly to your own needs.