I have tried to set a button with a different color background than the one defined in my theme.rc file with this command :

if not bouton.style:
                bouton.create_style()
bouton.create_style()["bgcolor"] = { Constants.STATE_NORMAL : _bg_normal,
                  Constants.STATE_ENTERED : _bg_entered,
                  Constants.STATE_ACTIVE : _bg_active,
                  Constants.STATE_INSENSITIVE : _bg_normal }

The background of the button gets the right color, but the background of its text is still grey (as defined in theme.rc). I have tried to change its child color with :

bouton.child.create_style()["bgcolor"] = { Constants.STATE_NORMAL : _bg_normal,
                  Constants.STATE_ENTERED : _bg_entered,
                  Constants.STATE_ACTIVE : _bg_active,
                  Constants.STATE_INSENSITIVE : _bg_normal }

But now every buttons have inherited the new color !

Cheers and have a nice day.