Let me remind you that you're looking for a custom forum style that was probably made over 10 years ago. Probably closer to 2006.
There is nothing difficult about emailing a forum owner about the source of their custom style. I mean, that's your original goal, right?
A quick Google search returns a post on ubbdev.com from 2009 where someone else asked the same question as you, and was given the same answer as I just gave you.
https://www.ubbdev.com/forums/ubbthreads.php/topics/317750/where-can-i-get-these-stylesheets.html
