You can edit (or create) the file named ".gtkrc" in your $HOME directory.
The file format is something similar to a HTML stylesheet, but it can get fairly complex.
The basic syntax is something like this:
[tt]
# DEFINE A NAMED STYLE
style "MyMenu"
{
font = [/tt]"-adobe-helvetica-medium-*-normal-*-18-*-*-*-p-*-iso8859-1"[tt]
bg_pixmap[NORMAL] = "<none>"
bg_pixmap[ACTIVE] = "<none>"
bg_pixmap[INSENSITIVE] = "<none>"
bg_pixmap[PRELIGHT] = "<none>"
fg[NORMAL] = "#ffffff"
text[NORMAL] = "#ffffff"
text[INSENSITIVE] = "#ffffff"
fg[INSENSITIVE] = "#000000"
fg[ACTIVE] = "#ffffff"
text[ACTIVE] = "#ffffff"
fg[PRELIGHT] = "#ffffff"
text[PRELIGHT] = "#ffffff"
fg[SELECTED] = "#000000"
text[SELECTED] = "#000000"
bg[NORMAL] = "#5F7EAB"
base[NORMAL] = "#FFFFFF"
bg[ACTIVE] = "#587296"
base[ACTIVE] = "#E5E5E5"
bg[PRELIGHT] = "#5F7EAB"
base[PRELIGHT] = "#FFFFFF"
bg[SELECTED] = "#A0FEEB"
bg[INSENSITIVE] = "#5F7EAB"
base[INSENSITIVE] = "#5F7EAB"
engine "gtk" {}
}
# ( MORE STYLES HERE ... )
# SPECIFY THE WIDGET CLASS THAT THE STYLE APPLIES TO....
widget_class "*Menu*" style "MyMenu"
# ( MORE WIDGETS HERE ... )
[/tt]
Note the wildcards around [tt]"*Menu*"[/tt] in the widget class -
It will match any widget with a classname of Menu, SubMenu, MenuThing, etc....