Apparently the new top bar rollout had some negative effects on the top bar of the network site stackexchange.com. It looks completely unstyled:
Since I see this on different browsers and machines, I assume it's a server-side issue.
I don't know exactly why it broke, but it is related to the earlier rollout of the new top bar code in Q&A. AFAIK, stackexchange.com is on the list to be updated as well, but that part hasn't been implemented yet.
I'm not familiar with what all changed as part of that project, so I'll bug folks to take a look tomorrow. But, since understanding the code before making changes is really more of a nice-to-have than a requirement... I'm pushing a fix to restore the styling in the meantime.