If you are using non default theme in Mint (for example Mint-X-Aqua in my case), you should change the corresponding menus.rc file located in the same folder.
/usr/share/themes/Mint-X-Aqua/gtk-2.0/styles/menus.rc /usr/share/themes/Mint-X-Teal/gtk-2.0/styles/menus.rc etc...
If you are using non default theme in Mint (for example Mint-X-Aqua in my case), you should change the corresponding menus.rc file located in the same folder.
/usr/share/ themes/ Mint-X- Aqua/gtk- 2.0/styles/ menus.rc themes/ Mint-X- Teal/gtk- 2.0/styles/ menus.rc
/usr/share/
etc...