Thank you so much for the snappy response! I see now what you mean by them just being sprites I can customize; somehow I missed that there was a folder called "gameframe" that got added to the project that had sprites in it in addition to the folder called "GameFrame" that was added to the project and had the scripts, etc. in it. I've never used a GameMaker extension before so that might be why; either way that was a really obvious question to ask now that you've explained it to me. Sorry!
As for the windows 11 border, I like that suggestion of creating a different border sprite and supplying it based on the os_version that GM reports. Thanks for that idea!
This is a great little extension and I'd definitely recommend it to anyone who could use it!