It doesn't work. The above again only changes the Menu Content Width (which is the value of Content Width in "YT Menu Params for this Menu Item".). That means the dropdown part of Mega Menu.
Anyway, I worked it out. The whole menu width should be changed in template.css. div.yt-menu (set as 73%). The gap between menu items should be changed in menustyle.css. ul.navi li.level1 (margin set as 0 15px 0 0).
I got what I want now. Thanks anyway.

Dear Hoang Chu,

Please send the screenshots to me if possible.

Thanks

You can find the result on the site I'm working on here
hoanganhtravel.com.au/test/