That would be my fault.
Winstep Xtreme is not hard to skin at all (especially not when compared to the hundreds of objects you need to skin in WB) and it used to have a built-in GUI for making themes.
Some years ago there was a total UI make-over and I decided to remove the skinning settings - very few people make skins, so those options were mostly clutter to the vast majority of users. The idea was to make a stand-alone theme builder some time later.
In the mean time existing skinners could use the old v9.5 version to make new themes or even edit the configuration files directly. Because of this, the motivation for me to actually do the theme builder was not very strong.
I did come up with an early alpha of the Winstep Theme Builder that could already make native Nexus docks skins (i.e.; it could only be used to skin the dock) and I asked for feedback. Unfortunately existing Winstep skinners at the time did not seem very excited with it and I ended up not bothering to develop it further.
Another reason: one of the very first Winstep applications was NextSTART (today a component of Winstep Xtreme, but back then it was sold stand alone) and 20 years ago there were a TON of skinners making themes for it at Skinz.org. In fact, a lot of well known skinners started their skinning careers making themes for NextSTART (for instance, Treetog).
What happened shortly after was that Stardock and WB became extremely popular and Stardock started developing applications that competed directly with Winstep applications (ObjectBar vs NextSTART, for instance). This completely fragmented the market and helped preventing Winstep from reaching the critical mass required to enable it to start hiring professionals to make skins for Winstep applications, create its own themes site, etc... Not blaming Stardock, that's just how it is.
Me taking years to finally come up with a dock (Nexus), although one was planned from the very beginning, didn't help much either, so...
And here we are now. Only a handful of skinners are left as far as I know, and most are still making skins exclusively for WB - and the Winstep Theme Builder is STILL not finished (working on it - vs. adding new features to Xtreme and Nexus - is extremely boring to me).
This said, the upcoming v18.8 release required such a mind blowing amount of work and took so much out of me that I will probably go back to the Theme Builder now, just to rest my head for a while.