Hi there, thanks for the feedback. I'm not a Unity user personally, but this asset is usable in any engine in which you can create a 9-slice, or through manually programming 9-slice behavior.
I'm aware that Unity can only create 9-slices when the images are packed together into one sheet, so this would require some manual effort.
For example, let's say you want to make a window using the theme shown in the banner of this page. To prepare a 9-slice for the part of the asset you want to work with in Unity, navigate to the window/light folder. Open the 9 files representing the 9-slice in an image editor. They're all named according to the part of the 9-slice they represent: window_top_left, window_top_center, window_top_right, etc. Combine them into one image in your image editor and save it. You should now be able to import the combined image into Unity and follow the manual for the rest: https://docs.unity3d.com/Manual/9SliceSprites.html
I've been intending to update these GUI asset packs with combined 9-slice files, so please stay tuned for future updates. As a Unity user: Other than combining the images, is there anything else I can do to make it easier to use? I would appreciate the insight.