Everything looks OK, except in your 4th picture the userChrome.css file is shown as a Text Document. On my W10 it's shown, correctly, as a Cascading Style Sheet Document. I don't know if it matters on a Mac, but that may be preventing the file from being properly read as a css file. When a new text file is created in Windows, and the txt extension is changed to css, a warning prompt is displayed about changing the extension etc., which can be ignored.

I've sent you a link to a userChrome.css file that has been tested and displays a 25pt line spacing.

I've attached a screenshot of the default Thunderbird look on mac. This is what you should be seeing before any customization.

re:4th picture the userChrome.css file is shown as a Text Document. On my W10 it's shown, correctly, as a Cascading Style Sheet Document.

Agree.

Have you got extensions set up as visible because that file may be called 'userChrome.css.txt'.

Please see if the info at this link helps you to make all file extensions visible. Then you can check to see if the file really is called 'userChrome.css.txt' when it should be called 'userChrome.css'. https://www.idownloadblog.com/2014/10/29/how-to-show-or-hide-filename-extensions-in-os-x-yosemite/

It does appear that the problem may be that TB is seeing the userChrome.css file as a text file - but I don't know how to change that. When i look at the full file name with extensions, it doesn't end in txt, but when I click "Get Info" it is described as a text file.

Any thoughts would be appreciated.

Here are a couple of links related to the issue mentioned above about unhiding extensions for known file types:

https://stackoverflow.com/questions/23976219/how-do-i-save-a-textedit-mac-file-with-a-custom-extension-sas

https://apple.stackexchange.com/questions/11135/how-do-i-create-a-file-with-a-custom-file-extension