The stock styles can easily be imported to your current site, and are located within your original ubbthreads-7-5-x.zip file.
for style "ubbthreads"
ubbthreads-7-5-x/install/styles/1.txt
for style "ubbthreads-light"
ubbthreads-7-5-x/install/styles/2.txt
for style "ubbthreads-dark"
ubbthreads-7-5-x/install/styles/3.txt
If you no longer have your original install files, they can be redownloaded from the"Downloads" link within the Member Area:
https://www.ubbcentral.com/members/members.php