My primary DNN host server bit the dust, so I restored from my backups to a new server.  Everything is working except the skins.  Basically, most of the CSS is not loading and neither are the images.  I dug a bit with developer tools and found that, for some reason, all the css and png files (maybe others, but those are the most obvious) are redirecting to /Account/Login which results in a 404.0. 

So if you look at the HTML you might see...

/Portals/_default/Skins/AvianWaves/DNNStandard/Images/rootItemL_Hover.png

If you browse to it manually, the browser redirects to...

/Account/Login?ReturnUrl=%2fPortals%2f_default%2fSkins%2fAvianWaves%2fDNNStandard%2fImages%2frootItemL_Hover.png

which gives you a 404.

Now, here's an interesting twist.  If you log in to the site, the skin works.  So it's only if you are browsing anonymously.  But my site has always been configured for anonymous browsing.  I'm guessing there's some small config issue with restoring from the backup, but I can't figure out what.