Reduce width of the entire portal page

Hi All

I'm trying to modify the width of the entire portal page (this includes masthead, TLN, content area) from 100% to 75%. I change the width property in the used css class, but the changes are not reflected.

If I have to change the width in the jsp page, which par file should I change? I checked the light_franework.jsp in com.sap.portal.layouts.frameworktest.par, but it does not have any html tags at all. Im looking for a table which uses prtlHeaderCon as the css class.

WIll be thankful for any help.

Thanks

J