Before Mary pointed out that I shouldn't be doing this, I altered my Standard theme. Now I am trying to untangle the mess and I need to put the code in Standard theme back to what it was. But Standard theme isn't available in the Themes Repository as it is available on all Moodle installations. (Yes, well, they didn't take into account blundering novices like me!).
I have narrowed the problem down to being in styles_color.css, in this bit of code:
I wonder if someone could kindly post this piece of code in its unaltered state or direct me to where I can find it.
Thanks so much.