GUIButton Standard Texture?

Hello !! If i create a GUISkin there's a standard button texture. as i inspected it and saw no real texture attached. there is a very small... something. The Question is how can i make similar ones? i mean the GUIButtons are quite cool but sometimes i want them in other Background colors or a little less transparent or something like that and i wanted to ask how i can make such own button textures !!

I'M not really good in designing textures or... designing anything.

i really hope you can help