Inteface decoration tutorial

Hi, I am sorry, I might just be blind but I´ve spent hour and half looking for answer. Is there any source of information how to create more complex decoration in interface? For example I would like to have canvas (or border) that has round corners and drop shadows. Let say I prepared textures for that, that means 4 corners, I know fill color (or I can use and repeated 1x1 texture) I have repeatable texture for every edge (so 4 textures, x pixels long, 1 pixel wide as I just want to repeat them along the edges)… simply put, textures sliced like that:

Now, I know I should probably use image and that there is Draw as border / box in appearance tab (while “box” for some reason seems to scale alright, “border” scales with “zoom level” for whatever reason). Question is, how do I assign specific slices to given areas? Also is there way how to use one corner texture, one edge texture and just flip / rotate them? Or do I “simply” have to construct those menu underlays from 9 images?