The theme is optimized for MacOS. If you are on Windows or Linux, you might run into some issues. I don’t have the ability to test this myself, but if you fiddle with the CSS you might be able to come up with a solution. The easiest is to comment out the “Frameless” section of the code.
If anyone reading this is using Windows/Linux and would like to assist, I’d love some help styling the window buttons so they work well on those operating systems.