Over the last month Google has decided to ignore popular opinion and institute an awful user interface. Luckily this is easy enough to f
All you need to do is make a bookmark to this location: Gmail Revert (drag link into favorites or right click copy link location and then save as a bookmark)
Then log into Gmail. Hit the bookmark from Gmail and you will see a notification stating that “Settings are being updated.” Now click on the gear in the upper/left and select revert to old theme.
Very easy and affective. It is likely that this will stop working at some point, hopefully by then they will have instituted an official classic theme.