I hop over to DeviantArt and look at skins compatible with that free skinning program and guess what I see? Creative Commons licensed themes everywhere. They're also all much better looking, better at keeping up with current design trends than anything free or paid that I can find on WinCustomize. The majority of the stuff I see on WinCustomize looks straight out of the year 2002 when 3D on desktop UIs was still the flashy new thing and 12 years olds liked to crank it up to 12 by slapping those silly eye bleeding skins on their desktop to make their computer look "cool". It's like you people have been living in a bubble here, completely unaware of the trends happening out there, thinking your stuff and your way of doing things are still relevant in this year of 2019. You guys need a reality check.
You're here rambling about all the time you spend on your skins and that it's unreasonable to expect any of you to agree to derivatives on your work because of the time and efforts you put into them. I spend days on inventing and modeling practical objects and tools for 3D printing and printing them several times as I iterate through designs, then once I'm done, I share those models under a CC BY-NC-SA licence on Thingiverse. I get a kick out of my stuff getting lots of downloads because it means I've had a good idea and that I've created something that is useful and helpful to people. When someone derives my work, I have a look at it and get inspiration from it to improve my own design if I feel I can do it even better, or let them take over the design if I can't do better. Sometimes people ask for variations with different measurements, which I happily provide when possible. THIS is the major trend right now and there is a reason this is the trend. Why don't you go and have a look at Thingiverse, so you can get a taste of what it's like when people who don't have a distorted sense of the value of their work and an ego to stroke create stuff and put it out there for everyone to use for free and to derive work from. It's great. Creative juices flow. Everyone benefits. Hell, even Autodesk has seen the light in that matter. They provide Fusion360 for free for startups and hobbyists.