I think that the Simple Jack theme was created by a Reaper user named Lamda.
Visit the following website:
http://www.mowerx.de/pages-reathemes/
Three weeks ago I sent donations to Lamda and received his excellent Premium themes in return. I also asked him for Simple Jack and he was so kind to send it to me. You may want to do the same thing.
Each theme of the Premium Series is excellent. My favorite is "Dusty". Although the name does not imply it, this theme is extremely clean and organized. You should also have a look at "Adapted". A simple theme, 2D, and much like an improved version of Simple Jack.
I personally find Simple Jack nice to have and I will keep it in my folder. But Lamda's Premium themes are much better and are the ones that get used.