It actually wouldn't surprise me if the buttons in Google Groups were com.google.gwt.widget.client.TextButton-s or similar, with a Groups-specific Appearance.
Decoration.PRIMARY would be red (new topic or post reply) or green (post) depending on context.
That button is the only (built-in) one supporting "collapsing" (IsCollapsible) as used in the overview/discussion or tree/flat "switches" (mutually exclusive pairs of toggle buttons), or the "post reply" and its "split button" to open the drop down menu; so it might very well be that they're using them, and maybe created a few others that haven't been open sourced.
On Friday, June 29, 2012 5:13:10 AM UTC+2, July wrote: