How to reduce the width of the drop down menu under GK Extra Menu ?

I am using Joomla 1.7: Template Corporate 2: Firefox 4.
Currently I am using Menu Type: GK Extra Menu
I am using firebug to inspect various elements and modify to suit my need.
I have noticed that width of the drop down menu is 200px which much more than what I need. I need only 160px hence I have done changes in the following file: menu.gkmenu.css

div.gk-menu > ul.level0 > li > .childcontent .gkcol {
float: left;
width: 160px;
}


However I can see that this width: 160px is being overwritten by to 200px but I do not know what file overrides this.
Task: I basically want to reduce the width of the drop down menu from 200px to 160px. How can I do that please?

Regards