Try in userChrome.css:

table[is="tree-view-table"] td {

 border-bottom: solid 1px white !important;

}

Thank you, Ed. I tried this, but no lines in the message list. Does this code need to be preceded by a namespace statement, or something else?

No namespâce statement. Did you restart Thunderbird after adding Ed's code to your userChrome.css file? Try the following CSS code in your userChrome.css (works for me on macOS)

tr[is="thread-row"] > td { outline: 0.2px dotted black !important; }

Thank you, Mapenzi, this works. I changed dotted to solid and black to white, since I am using a dark theme.

Now, I would like only row separators, no column separators. I assume I have to replace "outline" with something else. Would that be "row"?

Also, the lines seem very thick/heavy. I tried reducing the pixel size to 0.01px, but to no avail. I am not using a very high-res monitor. Could that be a limitation, or is there a way to reduce the pixel size.

Again, thank you!

Well, now I returned to Ed's code, and it works. Apparently I had a commenting error farther down in the userChrome.css file. The lines are still a bit too thick/heavy; the ones in Outlook look better, but these are acceptable.

Thank you both!

Oh, Marpenzi, I still would be grateful to know what to replace "outline" with. :)

You could use my code and replace "outline" with "border-bottom" . Sorry, I can't stand dark themes! The following code ....

tr[is="thread-row"] > td { border-bottom: 0.1px solid black !important; }

.... looks like that image in my TB

Thank you, Mapenzi! We can work with this example. I can't imagine working without dark themes, and my colleague can't imagine working with them. Freedom is good! :)

Are we using some standard css syntax, or something specific to Mozilla, or …? We would like to become more proficient using css with Tbird and also Firefox.

Are we using some standard css syntax, or something specific to Mozilla, or …? We would like to become more proficient using css with Tbird and also Firefox.

I think this is standard. Some websites : https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_syntax/Syntax https://www.w3schools.com/css/css_syntax.ASP https://www.geeksforgeeks.org/css/css-outline/ https://www.sfu.ca/~tutor/techbytes/Dreamweaver/dw2.html https://brainstation.io/learn/html/css-syntax

You can't make a border less than 1px. Instead of white, try #e6e6e6 or #d9d9d9.

You can't make a border less than 1px.

Are you sure, Ed? Here are two screen shots border-bottom 1px vs 0.2px ...

Thank you both.

Ed, nice softening of the white. I also gained some ground by changing the density setting.

Mapenzi, thank you for all those links. We will pursue them!

Withing you both a wonderful weekend!

Oops, that was supposed to be "wishing". :)