ModxMinify

MODX Minify is a MODX Revolution Extra to group and minify your CSS, SCSS, LESS and JS files. Grouping and minifying your styles greatly improves your page load time. You can create groups, and each group can contain multiple files of the same type (CSS (also SCSS and LESS or supported) or JS). ModxMinify automatically creates a minified file for each of your groups or combination of groups.

Install the extra through the MODX package manager. After installation a new 'MODX Minify' menu item will appear under the 'Extras' menu in your MODX installation.