How to (safely) change the grid

I'm building a Montezuma site, but the fixed-width grids available are a tad narrow, so I changed the width to 1200px by successfully hacking the css file in /wp-content/themes/montezuma/admin/default-templates/css/grids.

However, upon updating the theme, that stuff will inevitably get overwritten. So I came up with a brilliant idea to move the grids outside the Montezuma directory and do a good-old 301 redirect on a server level using an .htaccess file.

Brilliant if it worked, that is. I tried it, and nothing happened - the theme continued observing the CSS grids in their native location.

Anyone have any ideas how I may be able to move the grids while maintaining the updatability of Montezuma?

As in, move the grids directory and force the theme to look for it elsewhere without modifying anything within the theme directory itself?