The list doesn’t have any way to specifically target a Group.
But there may be a suitable work-around.
If a Category is specific to a Group, the Category could be targeted.
Rows have “first, last, frequent” poster information. So member names could be targeted.
A Group-specific Tag could be added to a topic and the Tag could be targeted.
I’m not sure about how well CSS could be used since there is no such thing as a “parent selector” but maybe someone better at CSS could figure out how to do it.
And I guess some JavaScript code would be able to do some magic, but I fear it would be quite a drain on resources running on every topic in a list.