Sure you can actually do this with some simple CSS.

For example the Admin class will be used in the topic post section specific to their post, so when you inspect a post made by an Admin or Mod or whoever, you should see in the code, looking at spTopicPostSection:

     class=”spTopicPostSection spEven spType-Admin spRank-admin

You can then create new entry in your theme such as:

     #spMainContainer .spRank-admin .spPostContent p {
         font-weight: bold;
         background: red;
     }

As an example, will make an Admin’s text in a post appear with a bold text on a red background.

You can do this for any Group as long as you know what rule to include (in this case .spRank-admin) in the CSS