Hi ronpat - pls excuse this delay in getting back. Lots of distractions at the moment...
Ta for toubleshooting the media query issue. You were quite right, of course. I added a second curly closing bracket and all is now well.
I make my code mods in my Wordpress child theme Dashboard CSS stylesheet. I've since established that the Stylesheet doesn't provide line numbers. I often paste code into Notepad to save it initially, then subsequently copy and paste it from there into the WP Stylesheet, but I've never seen any line numbers in Notepad, either. There's some way to switch them on so they display, I'm guessing?
I was dimly aware that line numbers of a website page can be seen using Firebug. I've set aside some time over the weekend to do a Firebug tutorial so I can get better acquainted with it.
Have I answered your question about the environment I'm working in? If not, not sure what you mean. :/