Duena entire theme width

I am using the Duena theme and I am trying to change the maximum width it can be when the browser window is fully open, I don´t want it to take up so much of the page..I want it to be around 75% (much like http://www.nciku.com)

I have read the other posts on this problem but they are unfortunately unsolved.

Do I have to change many different lines of script and files or is there one universal piece of code to change this?