Okay, so not a TDE thing!  Good to know.  The MX is currently a complete clean 
install as I haven't migrated anything over to it yet (just fyi).  I'll make 
backups and futz with the schemes and also search/ask on the MX forum.  Ah, 
yeah, dumb me, I'll check the "XFCE desktop" MX installs by default too, 
which I should have done before posting :(

When I get a definitive outcome I'll post back here.

Best,
Michael