First I would test that userContent.css is set up correctly by adding this line that colours the scrollbars in the message pane:

:root { 
    scrollbar-color: grey orange;}

or this, to colour quoted text:

/* Quoted text colour for plain text */
span[_moz_quote="true"] {
  color: navy !important;
  background-color: mistyrose !important;
}

pre[_moz_quote="true"] {
  color: navy !important;
  background-color: mistyrose !important;
}

As simple as it is effective. I inserted the second part of your CSS in my userContent, saved it, stopped TB and restarted it. Quoted text is still indubitably white.

I searched the config editor; devtools.chrome.enabled is enabled by default, I enabled also devtools.console.stdout.chrome but nothing changes. Restarted TB various times and found nothing relevant in the console window. Ran strace and found no call with the string "userContent".

What is preventing it?

You have to follow these instructions:

Help/Troubleshooting Info, Profile Folder, Open Directory (Win:Folder), close TB, create a new folder named chrome, create a new document in chrome with a text editor, name it userContent.css, Save as type: All files *.*, copy in the above code. Double-click toolkit.legacyUserProfileCustomizations.stylesheets to true in Settings/General/Config. editor, restart TB.

Online css generator

video on how to create a css file (Firefox and TB)

Check that the toolkit pref. is true.

I ran TB under strace and clicked that Open Directory button. Then I found this line in strace output: 29059 access("/home/ale/.thunderbird/bmvmhlad.default/chrome/userContent.css", F_OK) = 0 TB only checked the file is accessible. It didn't open it.

In the video, a folder view opens after clicking the button. At mine it doesn't. But the terminal displays this: Traceback (most recent call last):

 File "/usr/local/bin/catfish", line 43, in <module>
   import catfish

ModuleNotFoundError: No module named 'catfish'

So I installed catfish. It's not very useful, as it searches files for the content one types, in all subdirectories, including myriads of mail files.

I can't help with the Linux dialog, but if you have the css file in the correct place and toggle the pref., it should work, as it has for other Linux users.

I switched devtools.chrome.enabled to false and then back to true, so that it is explicit in pref.js.

I also tried Troubleshoot mode.

There must be something which prevents userContent.css from being read.

Maybe some leftover, although I searched /etc/thunderbird and found nothing relevant. There are Debian settings from 2018...

Troubleshoot/safe mode won't help debug a css issue, as userChrome and userContent are disabled by default in that mode.

The only pref. that needs to be changed is toolkit.legacyUserProfileCustomizations.stylesheets.

It was false by default. I turned it to true and restarted. I still don't see a mistyrose background, but text inside <pre> is finally wrapped!

Thank you.

If your default message composition style is HTML, hold Shift when clicking Reply so the format changes to plain text. In that case, the quoted text will display navy on mistyrose (see picture).

My default style is text, but the long-pre message I used for testing was HTML. Afterwards, I found out that mistyrose does appear in the background, and I immediately removed it from my userContent :-)