Sorry, I meant to say you can change the actual theme that you have to 960px rather than use an entirely different theme.
If you look in your theme in the first responsive section, the media query should be 1139px (just do a search within the file) Increase the number to like 5000 (or whatever is enough) so that it will always be 960px no matter what size the browser is….unless someone is viewing your site in a movie theater 😉
So that would mean you can use the latest version of the theme (recommended) and still use it at 960px. Does that make sense?