I use the Ambiance theme which is nice except this horrible orange everywhere. To change it just type in a terminal:

gsettings set org.gnome.desktop.interface gtk-color-scheme "selected_bg_color:<your color in HEX>"

or use the gconf-editor tool and browse to: desktop > gnome > interface and change the value of gtk-color-scheme with selected_bg_color:<your color in HEX>