Combine alpha of GUI style texture with icon texture

Hello, can anybody help me with my problem?

I would like make some GUI button behavior. It is combine alpha channel of GIUStyle background texture with foreground GUI.Button() texture. This behavior is typical in many platforms but I can't use it in Unity. For example I would like to show rounded corners in foreground texture but use data from Style background texture. It is because of 9-slice techniques. You can see it on uploaded image what I need. Than you so much.