Hi! to add antialiasing, you have to reimport the textures in godot import tab, checking the "filter" option. Is that what you are looking for?
That works! Unfortunately some of the elements, like the checkbox, are too small to see, which isn't your fault, but mine since I tried to use a pixel-art theme with a 720p game. I might end up using your SimpleBox theme instead, with some modifications. If you ever decide to do a larger/HD version of this ornate theme, though, I can guarantee you one sale! :)