Buddypress Group CSS plugin by rustybroomhandle alows a Buddypress group administrator to customise their group with CSS via the “Group Custom CSS” tab under the group’s Admin page. Also supports preset themes.
The author says “the css in the code is only really garaunteed to work with the default Buddypress theme, and it is assumed that you as a site builder will customise it to fit your site’s needs.”
Download: http://wordpress.org/extend/plugins/buddypress-group-css/