Not sure how I missed this above post, but I'll respond to this now...
I think JRiver should offer skins for different OS's
This is likely up to the skin developers. If you look at the main.xml files for every skin, you'll notice every skin bundled with MC was created by a third-party, even Noire. That said, there has been skin updates over the years thanks to Matt, Hendrik, Marko, Dirhael, me, etc. including newer Windows buttons for Noire, the multi-OS support, scaling support and new tag window support.
I do offer skins for some of the Linux desktop environments. I also forked off Modern Cards and created the Modern Cards: Grey Edition skin which is purposely Mac-esk and is included with MC. I did intend on also creating a "dark mode" version of that Mac skin too but never really had much time to invest in that (though I assume the Dark Edition and Light Edition skins are kinda close to this).
What's on my skinning wishlist to make the skin more Mac-esk? Two things... 1) OpenGL support for glass/transparency so I could attempt getting the "frosted glass" interface working on macOS and 2) when hovering over one of the three stoplight (the close/minimize/restore/maximize) buttons it should cause all three to go into "hover" like it does natively in macOS instead of just the stoplight button it's hovering on.
If I get some extra time soon (probably after I've done a new Windows 10 Defender taming guide) I might try updating at least the Purity skin for macOS (or forking off a custom version for you, which probably would be easier).
Even if I replace the buttons, the skins still have rectangular edges which does look horrible on macOS (at least on the non black skins).
Looks like a few skins (e.g. Blue Steel) have rounded edges (looks like this is done via Frame_LeftBorder and Frame_RightBorder), so this could be doable. I might look into doing this someday with the Modern Cards: Grey Edition skin.
Also I'm preety sure the buttons and their arrangement follow some apple guidelines (in the attached screenshot you can see buttons of a Finder window and MC). Another thing is: why are the buttons colored when I explicitly changed this option in macOS settings to grey? this is just another sign of a bad integration into macOS..
MC doesn't use the native stoplight buttons, hence why they don't change to grey. The stoplight buttons you're seeing in MC are individual images that were ripped by me from macOS. I never bothered grabbing the grey versions of the icons since I didn't honestly see any need to. Since MC uses its own system for window drawing, it knowing to switch to the grey buttons when changed in macOS itself as it is right now probably wouldn't be possible, even if I ripped those images from macOS and created grey versions of the stoplight buttons.
Honestly unless you can modify/create/improve skins yourself it's probably as good as it's going to get.