There was much new code (part of the goal) in 115 that eliminated much of the CSS that many (including myself) have used through the years, although such CSS has never been supported by Thunderbird. Discovering replacement CSS with the new environment will be a matter of individual experimentation. SO, what that means is that some of the CSS tweaks that we have used may not be available in the near future, if at all. CSS is no specialty of mine so I also am waiting to see what new CSS becomes available.

OK David, thanks for your reply. I'll just have to keep an eye out for advances on this front - not being very technical doesn't make it easy though. Cheers

I can give you a solution for zebra striping, if that would help.

OK David, thanks for your reply. I'll just have to keep an eye out for advances on this front - not being very technical doesn't make it easy though. Cheers

"not easy" is one of the reasons why modifications via css is not recommended. It's not particularly intended for end users.

I understand why developers don't want novices messing with their programs but, once you find something which enhances your experience of the program, it's difficult to give it up, as you know.

Yes, zebra stripes would be a step forward, please!

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 and place within chrome folder
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;  
}

I just use the light theme and this works fine - much appreciated.

I guess horizontal lines/zebra striping just aren't much in demand so are not built-in as options.

Cheers