The default window border sprite has a little black-white border - I think it’s not visible on the itch screenshot because the window doesn’t have focus (I don’t have any Win11 devices myself so that’s a screenshot provided by someone). You could supply a more fitting one (either with a rounded border or just no border) based on os_version.
Also, is there a way to set the color of the individual parts of the interface? Say I wanted a black background with white text and button images. The cheatsheet references changing the interface color with something like “gameframe_blend = c_black”, but doing so colorizes all of the window UI elements at once and not the individual font color or button color. Is there a simple way to control that?
Caption background is also just as sprite that you can customize, but you also can override drawing via callbacks. Button drawing has its own callback that you can change.