Changing the Max Page Width of a Responsive Theme

Hi Guys

I am just getting back in to WP after quite some time away from it. I have chosen blog-64 as the theme I want to use. It is a responsive theme which I am OK with, but I just want to make sure that the maximum page width will be about 1500 px.

Reason for this is that I have a 30-inch and 24-inch monitors and when I get to a site whose page width is set to 100% and it takes up the full width of the monitor, I really don’t like that view. I would prefer a view about 75% of that for any sites I build in WP.

I can set page width via px or %.

Can anyone suggest the syntax and css page to change the width in please.

I have a question in to the theme author but it may take some time as he is on the roof of the world someplace.

Thanks for any help.