You cannot customise the ID (the number), as its an ID. If you are looking to add CSS to a group name, you can actually do that directly within the software. If you go to Members>Groups and edit any group, you will see 'group formatting' where you can choose a color. However if you wish to do more than this, such as wrap it in span tags so you can style that way or even add a CSS class, you can do this by selecting 'Use HTML' at the side of that setting.