If I did have any desire, it would be extinguished by the license terms, lol. The terms are completely absurd in the way they are written, e.g. "You may not create, maintain, or distribute a forked version of the software." So arguably making any changes would be considered "creating a forked version." But even taking these terms as they are likely intended (which is slightly more permissive than how they are written), they are terrible. No thank you.
Similarly, to support Dark Mode as a Win32 application is difficult. There is no easy way to enable it. You can use an API to get your titlebars done, yes, but e.g. for menus and listboxes and other controls you're pretty much on your own! Which is stupid, they could make it straightforward, but it feels like they are trying to get people to use UWP instead so don't bother.
To answer the final part of your question - these APIs have little do with the GUI programming, which is what we use win32 as a common target for. There's no Linux equivalent of win32 for windowing. If you want to create UIs on Linux, you pretty much have to use a toolkit. So targeting win32 and then wrapping that to other OSes makes a decent amount of sense.