It doesn't take much Jass Knowledge. You could do it in GUI with custom scripts. There are 5 functions you need to know:
JASS:
New(real minx, real miny, real W, real H, real Z, integer Texture)
SetTexture(Button, integer Texture)
ShowButton(Button, boolean show)
AddActionL(Button, function)
AddActionR(Button, function)
These are all really simple functions.
New creates a new button
Set Texture, self explanitory
ShowButton, self explanitory
AddAction Left Click
Add Action Right Click
You don't need vjass knowledge. Thyrael began making DarkWind full screen systems with DGUI in GUI so you don't need any vJass knowledge.
For the skins, you could either use the wc3 borders, backgrounds, inventory slots, items, crusors or you could import like 6 2kb size textures with about 30kb use for model space. Not even 30kb, probably less.