At one time, along with PrE, came a subacription to Adobe.com, and for Plus Members (additional subscription fee), US (only) members could download some additional themes, however the list of offerings were not that extensive, and many people were dissatisfied. I do not know of that possibility now, and have not heard of any Plus membership in some years now.
One possibility, though I think that it might fall a bit short, would be to choose a Theme, and then edit that, changing thing about.
Another would be to go to Muvipix.com, as there are many custom Menu Sets, plus several active members there, have spent a lot of time changing, and customizing things in PrE. I do not recall seeing any custom themes, but I am certain that some Muvipix folk have tried, and can give you tips, and maybe even point you to other sources: www.muvipix.com
Good luck, and if you DO find some custom Themes, or find ways to create your own, please update this thread, to help others in the future.