Alternating colours try this: Suggest pale grey #f2f2f2

I'm assuming you were already using a userChrome.css file, so adjust using the code between the lines below.

#threadTree tr:nth-child(2n) {
  background-color: #f2f2f2;
}

Please report back on results. Remember to exit Thunderbird before adjusting userChrome.css file.

If you cannot remember/ do not know how to set up userChrome.css then please ask and I'll provide more info.

I was not using userChrome.css in TB 112. Alternating colours per message in the main pane were presented by default, but no longer in TB 115.

I created a userChrome.css using the css as described. I enabled toolkit.legacyUserProfileCustomizations.stylesheets. The main pane still has an all-white background.

Try this: For zebra striping, I show two options, one for white background and one when using dark theme.

First, if not already done, 
Go to settings>general, scroll to bottom for config editor. 
Search for  toolkit.legacyUserProfileCustomizations.stylesheets   and set to True 
Then in profile, create a folder named chrome, if not already there
Then, create a text file named userChrome.css if not already there 
Then, place one of the following in the file. Close TB if running and restart. Feel free to twiddle with the colors, but do not ask me for help in that. This is an area of personal experimentation. But it works. Good luck. 


/* for WHITE background ver 115 plus */
/*  #######    */
@namespace html url("http://www.w3.org/1999/xhtml");

#threadTree tr:nth-child(2n) {
  background-image: linear-gradient(rgba(0,0,0,.08), rgba(0,0,0,.08)) !important;  
}

########################## separator line - do not include ########

/* ####  for ver 115 version  and DARK theme   */

@namespace html url("http://www.w3.org/1999/xhtml");

#threadTree tr:nth-child(2n) {
  background-image: linear-gradient(rgba(255,255,255,.15), rgba(255,255,255,.15)) !important;  
}

Thanks...placing userChrome.css inside a folder named chrome resolved it!

Is there a way to detect dark vs light theme, so that this customization can be dynamic, instead of hard-coded to one of light or dark?

I was not using userChrome.css in TB 112. Alternating colours per message in the main pane were presented by default, but no longer in TB 115.

Not with native Thunderbird 102 unless via some theme.

snip

Many thanks - I wasn't actually looking for anything this advanced, but you gave me enough info to fix the message pane itself (which 115 is ignoring the system color scheme on and thus making unreadable for me).

I've created a new question which offers a load of info code for anyone wanting to get the zebra striping. https://support.mozilla.org/en-US/questions/1425551