AppStore formatting is broken

Try opening Terminal and entering the following command to enable the App Store Debug menu:

defaults write com.apple.appstore ShowDebugMenu -bool YES

Then, quit the App Store (if it was already open), and relaunch it. You should see a new menu after the Help menu labeled Debug. Open it and select Reset Application.

If that doesn't solve your problem, you might need to flush the DNS cache, the commands are listed below.

Yosemite: sudo discoveryutil mdnsflushcache
Lion, Mountain Lion and Mavericks: sudo killall -HUP mDNSResponder
Snow Leopard: sudo dscacheutil -flushcache

(to remove the debug menu from the App Store, enter the same command above used to enable it, and change the end from YES to NO.)

In case anyone's stylesheet is being blocked by a proxy, Apple seems to host the Mac App Store stylesheet at "s.mzstatic.com". I used the debug menu to download the page source and find out where the stylesheets were being hosted.

So, if you're facing network issues, ask your network admin to unblock *s.mzstatic.com* and you should be good to go.

I finally figured it out. In Keychain Access, I selected Preferences, then Reset my Default Keychain. After doing that, I restarted AppStore and it was working normally.

Warning You may loose other passwords stored in the Keychain, so make sure you have a backup of them