Modifying menu color of a gtk 3.0 theme

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http://unix.stackexchange.com – I'm trying to integrate a GTK3.0 theme with xfwm4 theme, Now I need to change the menu background color, so I get the color value with gcolor and tried to grep for that value, nothing found under the gtk-3.0 folder. What's the right way of doing it? (HowTos)