This would have to be added to the skin itself. I did post some older skins that better fit the Windows 11 (Acrylic) aesthetic here: https://yabb.jriver.com/interact/index.php/topic,106397.0.html
I actually do have newer Modern Cards skins (which I call the Mica series) that aim to better fit the Windows 11 aesthetic and they do have rounded corners when windowed, both in a light and dark variant. Unfortunately progress on the Mica series has been halted because of issues with transparency not working fully in MC currently. I was attempting to do experiments with transparency trying to get a closer match to both the Acrylic and Mica (and MicaAlt) aesthetics from Windows 11 but found transparency with images is partially broken (they appear opaque with no transparency) at the moment... except with the images the skins currently use which do work and is a bit odd. Anyways, hopefully someday that issue can be fixed and then maybe I'll tidy things up and post them if anyone is interested. The other caveat with the Mica series skins is there's no scaling support, I had to remove it as it was causing issues with transparency too.
Here's a screenshot of the light variant...
And here's a screenshot of the dark variant...
hi Awesome Donkey
wow , i like the Modern Cards skins , it should be the standard theme at the first installation , just because i have changed the program language
in short this one , taken in the forum , should be the default beautiful theme , the medium grey , sorry right now I can't check it
may i know how could be added some button at bottom ,under display ? i'm talkig about your screenshot
I really appreciate you help , sorry for my poor english and thank a lot for the skins