Seems to be a bug with caching, the site mixes both variants:
this is the first line in the cached CSS
/* http://www.elkarte.net/community/themes/default/index.css font-awesome.css _besocial/index_besocial.css _light/index_light.css prettify.css */
It seems like I wasn't the only one:
http://www.elkarte.net/community/index.php?topic=600.msg6846#msg6846
Sorry about that, I remembered this bug (where we had to turn off the cache) and was just doing some testing to see if it had been fixed .... I'd say no, and it is a block for B2
Updated the site with a fix and turned things back on ... let us know if you see the same problem any longer.