Thanks for that, Twinstream. It is extremely helpful.
I wanted
(a) to experiment in the same way with CSS Grid Builder as those videos do with tracks and blocks;
(b) to see the results on the main canvas, and not have to use 'Preview on...' on each change;
(c) to have each block numbered, but also have its own ID so that I could change the size and styles of individual blocks;
(d) to add or remove blocks with ease without messing up the numbering.
Hopefully others will come up with better solutions, but my own very simplistic method is
this CGB project file which I save as a theme so that I can easily reopen it in its initial state after experimenting.
Frank