remove bg_image from gtk-style?

[Edoardo Vigoni <vigoni aero polimi it>]

I want to create a button with the same shape of default button defined by the gtk-style in use, but with another colors (removing standard bg_image?). If I define a new style and color, the button appears with the classic rectangular shape and not with the theme one. I want instead the button with the shape of actual theme loaded on gnome, but with a different color. How can I do it? 
Sorry for my poor English! 
Edo