Changing the smartmenus breakpoint for Backdrop CMS

The breakpoint for a Smartmens menu to go from displaying only the hamburger to displaying the whole menu is 768px by default. I had a site where I needed the breakpoint to fall at 1120px instead, here's how I changed it:

I started by copying two CSS files from backdrop core into my theme. The two files were named menu-dropdown.theme.css and menu-toggle.theme.css and are located in the core/modules/system/css/ directory.