In layout editing mode, right-clicking vertical or horizontal splitters now enables setting a border. However, changes to borders don't seem to be saved as for me they are lost on a foobar2000 restart. Exporting and re-importing themes often, but not always, results in changes to border settings being lost. Tabs containers seem to have a different borders option and that seems OK.
Tested with foobar2000 1.4 beta 2 and no other plug-ins.
Fixed for beta 3, thanks for reporting.