Does CS-CART's backend use 960.gs? (MultiVendor v4.2.4)

I want to modify some look and feel of CS-CART's backend, but I don't know how start yet. I check the generated standaloneXXX.css, and it says:

The complete list of files it's generated from: design/backend/css/lib/ui/jqueryui.css design/backend/css/styles.less design/backend/css/glyphs.css design/backend/css/addons/help_tutorial/styles.less design/backend/css/addons/seo/styles.less design/backend/css/addons/call_requests/styles.less design/backend/css/addons/discussion/styles.less design/backend/css/addons/hybrid_auth/styles.css design/backend/css/addons/twigmo/styles.css

but I notice that there are some file like 960.gs and bootstrap in design\backend\css\lib

what is those 960 and bootstrap file used for?