Okay, that gets me back to a (slightly funky) black border around current cell, so: good (having updated to LO 7.3.6.2). Thanks!
As with all updates, there will be some getting used to new little weirdnesses, but I’m comfortable continuing with LO updates now.
However, as of LO 7.3.6.2 (at least on Macs), the path to the controls to effect the change are as follows:
LibreOffice > Preferences > LibreOffice > Advanced >
[Open expert configuration …]
then continue as in the original link:
“Put the single word TransparentSelection into the search box and search.
Double-click the line with TransparentSelection so the value switches from true to false.
Confirm dialogs with [OK]”