Trying to get site to use full width

Is there way to make the site use 100% width instead of saying how big you want it in px, has this is not a very good web practice as not everyone has a screen that can do 1950 or whatever you need the site to be set at to fit everything. I have been fighting with this template that my client has purchased and has his heart site on using this template, and help would be great.

No. All our templates has fixed width. We do not produce templates with variable (100%) width.