Changing The Width Of The Theme

“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 https://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?”

I was told to make my own topic. I have this exact issue. Can someone help me out?

My site is: rangolisweets.net

Thank you so much.