I dug up a thread on how to do this by using the forum's search, but I'm concerned many won't go through the effort. So...
To get the old GH theme ("more-or-less"), do the following (credit: strafe)
Go to Profile > Account Settngs
Then go to Modify Profile > Look and Layout
Then click on (change) to select a different layout.
Nostalgia is the one you want and I will personally cast my vote for it to once again be the default theme for GH. I know it's dark, but the colors really pop!
Yes, just like switch type, it's a matter of preference, but for those like me that want the old theme back - this threads for you