I have a new laptop running Buster (with the MATE desktop) and noticed my Emacs highlighted text was not showing any differently than non-highlighted text. After some conversation on the #emacs IRC channel, it was suggested I run this in a terminal window and restart Emacs:
$ xrdb /dev/null
Lo and behold Emacs highlighting worked again!
That lasted a day or two, then highlighting disappeared again. I repeated the xrdb incantation and Emacs highlighting started working again.
In the old Linux days I could deal with various desktop, X, and other windows configuration but I haven't had to deal with that for a very long time so, I am at a loss as to the next step.
One thing mentioned on the IRC: the contents of my .Xresources file. I had none, and one of my correspondents gave me a link to his, but I haven't used it. Instead, I created an empty version which has not affected anything. I wanted to see if the mere presence of the file would affect the situation, but apparently not.
I hope someone can suggest something to fix this problem permanently.
Thanks.
Best regards,
-Tom