unable to find where a class is defined in css files

My site is http://mobileknits.apps-1and1.net/ using twentysixteen theme.
I know how to use Firebug to locate a css rule and alter it. But a lot of the rules I just cannot find anywhere, for example I would like to change the background colour of the menu bar. Firebug points me to class=”menu-menu-2-container”. I have got the style.css file off my server and searched for that class but it’s not there. In the CSS tab of firebug there is a dropdown called inline. This shows loads of css files (some clearly related to my plug-ins) so I am thinking my missing rule is in one of those files, but where are these files located? The style.css and rtl.css are the only files I can see on the server. many thanks in advance, regards Dave