It's a shame there aren't more themes which provide zebra striping. I think that for the vast majority of users, a theme is by far preferable to the alternative. But I know what you mean about themes being not to your liking.

However, you can do it for yourself. http://kb.mozillazine.org/Zebra_striping

But I know it's not ideal. You need to be confident about working in the file system with hidden folders, creating a folder and using an editor to create a text file, That's quite a learning curve for many users. And any small error in your construction or syntax might just kill it dead. There is not much documentation on the format of Thunderbird's css rules.

You may need to shoehorn your changes into the file at just the right place. Of course it needs to be after any existing formatting that you want to defeat, and identifying any such isn't trivial either.

I've made this work myself, in the past, so I can say that I know it can be done. Don't give up, yet.

Thanks, Zenos.

I'm not giving up. Well, I'm not one who does. I went down that "rabbit hole" for about 6 hours this morning. I'm not willing to let it go, and I'm one who is well-versed in getting around in the file system. But what I keep reading is about the "chrome" folder and putting userChrome.css inside that folder. When I "finally" found the folder, added the css file, it did nothing. And I'm a Web developer by trade (more than 15 years). So creating a css file is nothing for me. Getting the "right spot" for the file to work is another venture. I've yet to find a great description of the "how". I have it in: C:\Users\name\AppData\Roaming\Thunderbird\Profiles\characters.default\extensions\{lots-of-characters}\chrome But, that doesn't seem to work. I know for a fact that it's the "only" chrome folder in any part of the Thunderbird file system. Whether or not there needs to be another is out of my pay-grade. I'm running Win 8.1 and pretty much everything I've found is prior to 8.1 which is a completely different file setup for the OS files. So, if "someone" could tell me the "correct path" for the chrome\user.Chrome.css file, I would indebted. An update - I switched "back" to using Eudora OSE and...Gmail works now. I did update Thunderbird, and that "may" have done something for me, since both EOSE and TBird use the same file systems. I know I want to fix the "zebra striping", but it's not as critical now. It's odd, but your reference to "zebra striping" was only the second time in my search that I saw that reference. Everything I found kept referring to the "thread splitting". Now that I know it's called "zebra striping" I may be able to get better information. Thanks.

go to the help menu, select troubleshooting and click the show folder button. Create your chrome folder in that folder (the root of your profile BTW) Or I could have said

C:\Users\name\AppData\Roaming\Thunderbird\Profiles\characters.default

Place userchrome.CSS and any Userchronme.xml in it and restart Thunderbird.

(now to figure out what I customized with my userchrome.XML.

The filename, at the least, is case sensitive and I suspect the folder names might be too. So,

chrome
userChrome.css

are required. I haven't encountered an instance of userChrome.xml yet. The chrome folder must indeed be at the top level inside your profile folder.

BTW, some styling needs to go into

userContent.css 

I can't remember if zebra striping goes under Chrome or Contents.