Pure & Simple content width

Hi,

I am using the Pure & Simple theme. Here is the website I am editing: nccit.org . When I adjust the window size, the content of each page adjusts but the borders and width does not remain consistent. What I would like is for the content of each post to remain the same width or at least remain consistent in relation to the margins.
Or, put another way, I would like for the width of my posts/the content of each page to be wider, in general.

Thanks, and let me what I need to clarify.

Mark