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.
Looks good, but I’m getting this:
Fatal error: Class ‘BP_Group_Extension’ not found in(/…/…/plugins/buddypress-group-css/bp-group-css.php on line 81)
I think I can work it out…
Thank you.
That plugin is only compatible up to wordpress 3.05.