The first line in the userContent.css section for any given URL should specify the domain (or domains) that the section is interpreted for. Commands that follow it are within encapsulating braces { ... }

I had, for instance, a userContent.css file already for this forum, so at the end of the file, I pasted the lines in for the google maps tweak, creating one big(ger) file with both sets of tweaks included. There is one line, at the very top of the userContent.css file, that is also repeated at the very top of the example Google Maps tweak -- it is not necessary to also copy/repeat that line in the file.