Previously I'd raised this without knowing enough to explain, but I've
managed to recover information about it to hopefully describe it better,
and also hopefully end up with a solution.

Back in the day (2014?), I'd used references from old Moz pages about
how to change the appearance using CSS, and followed the steps to copy
the example 'userChrome-example.css' to 'userChrome.css', and added a
few changes for msg highlighting and folder & thread panes background
color. These changes took effect as expected and have been in place
ever since, throughout the upgrades over the years, until recently. We
prefer the 'default' display theme as opposed to 'modern', and after
upgrading roughly 6-8 mos ago, the folder & thread panes background
changed from the solid pale green to alternating lines of pale green and
white. Changing the view theme to 'modern' returns the panes to solid
green background, but that's not what we wanted.

The other 3 CSS changes regarding msg highlighting work fine with both
views. I've added the green background CSS command to another PC with
the same SM version installed with no other CSS changes at all, and I
see the same green-&-white alternate lines behavior.

I haven't been able to find any reference as to what other call-outs are
available in the event that the ref I'm using has been deprecated, and a
brief attempt to use DOM Inspector has been unhelpful, to say the least.
Sadly, I'm not a programmer so don't have the experience to take this
any further. Can anyone guide me toward either A) documentation to show
how/why this can't work now, or B) a means of using the 'default' theme
with folder & thread pane backgrounds other than white? Where possible,
in the MS world I use dark mode at every opportunity, so those glaring
white panes in SM are less than desirable.