This is how you go back to classic.
Does your mail look like this?
It looks like this.
Go to Viewing Email then choose the basic setting under Mail Version and save.
Now if you have issues with it not going back to your classic theme and your using internet explorer then make sure you add that site to your compatibility list like I did with this client. See the difference below.
This is what hers would look like until we switched to comaptibility mode.
This is what it looked like after compatibility mode.
With IE open go to tools, compatibility view settings and add the url to the list and close
Computer Repair Lakeland FLwww.MyComputerGirl.com