How to restore visible region highlighting

[Richard Wordingham]

I've just jumped from Emacs 24.4 and earlier on 32-bit Ubuntu 16.04.3
to Debianised Emacs 26.3 on obligatorily 64-bit Ubuntu 20.04.2.

One of the things that broke is the highlighting of the 'region'.  My
default background colour is white, and region used to be given a
yellowish background.  It is now given a white background, as I have
found by making my default background LightGray, so I do have a
workaround.

However, how do I change the background colour used for highlighting
the 'region'?  (Transient mark mode is already enabled.)  I deliberately
chose to have a white background for normal text, and I'd like to
retain it.

Richard.