changing style in a button

[Edoardo Vigoni <vigoni aero polimi it>]

Hello! I want to put some button in a window using the same style configured on Gnome (border type, etc...), but with a different color (for example red, or green). My gtk-theme has a bg-image on button and rounded corner. If I create a new style with red color inserted, I create a classic red rectangular button (no rounded corner). If I use default style I can't change color (the button has the default bg of the pixmap selected by gtk-theme). How can I resolve my problem?

Sorry for my poor english
Thanks in advance for your time and assistance.
Edo.