AmiModRadio can't use skins the way AmigaAMP or AMPlifier make, but it doesn't mean we can't make our own backgrounds. I've searched for a boing ball, cutted it (it gives depth to the image) and then I've made the gray background in CorelDraw in a few minutes.
I've made this for 256 colors now:
Or 24 Bit:
You just have to open the MUI prefs and select Groups, Container, Backgrounds and then select the image
To change the look of the folders it's just to change (I use MUI 4) Groups, "Layout as" Buttons (instead of tabs).