Morning AD.... Finally got around to having a look at these...
I won't really use the dusk or light variants, so didn't do a deep dive into those. "Dark" may, or may not need attention, depending on your intentions...
Here's what I found:
The InPlaceFindToolbar is not being used, so MC pulls the white one from default skin items, which just looks wrong to me? I added to main.xml here, looks better.
The list group header is not being used. The required image is there, just not used. I think this one can come down to personal preference. Mine is to use it, so I added it to the main.xml here

The skin uses greyscale icons in the tree and action window, but then uses full colour icons in the options tree. Options.png is referenced in the <ART> section, but the file is not included in the skin folder. I added one here, and it's a better fit, to my mind at least. If you agree, I've attached mine here if you want it.
There's something not quite right with FrameTopBorder (the black border is too thick) but I've not had time to figure that out yet

The stock "Modern Cards: Dark" skin also has all of the above issues. And should probably be fixed? For a more professional look, feel and new user experience, default skins should really be as close to perfect as possible.