Well… no.
I’ve created a Theme called PrimaryColors, I use not for esthetical reasons but purely for testing what Palette and Qss settings have what effect in the UI.
I’ve annotated the Palette file with all the UI-elements a particular color setting is affecting. The default palette is also already annotated by the Developers, the Qss sparcely. The color and style names are somewhat descriptive.
The process of creating a Theme for Windows, I’ve described in mijn book.
Several posts in this Forum describe the way to create a Theme, which taught me a lot. But nothing beats hand-on experience and discovery.
There are some specs on the Qss and Q-? platform the program is buid on. I can look those up and post those here, but I believe the software also has to implement the usage of Theme settings for anything to work. AFAIK those specs are not available.
You can learn from other Themes just by unzipping and analyzing them. Windows Themes are found here: Scrivener 3 Themes (Windows)
So, there you have it. Until I write another book about Creating Themes for Scrivener, that’s all the specs we have.