Offer customized WPMU DEV themes for members

I want to provide pro themes for members, someone registers, and they are provided with the default theme...it looks good but not great...but what i want to offer is great looking themes if they upgrade i'm trying to figure out the best way to create the "great" looking themes maybe like 3 to start.

What's my best method to create the "great" themes? and resell them via pro sites?

Do i use themes from here https://premium.wpmudev.org/projects/category/themes/ as my "base" theme(s)?