In a collapsed thread format: Top email will be underlined when it contains an unread email in collapsed view. In expanded view old unread will be bold and new unread will be bold and also display star. The 'sort by read' column will display green dot icon.

To use bold:

• Menu app icon > Help > More Troubleshooting Information
• click on 'Open Folder' button

a new window opens showing your Profile folder.

• Close Thunderbird now - this is important.

• Create new folder and call it chrome note the spelling - use a lower case 'c' It should be in the same place as the 'Mail' folder.

• open Notepad:
• Copy all the text between lines below and paste it into the Notepad document.

/*
* Do not remove the @namespace line -- it's required for correct functioning
 */

/*
    * Collapsed read thread, but the thread contains unread , set text to bold black 
*/
   treechildren::-moz-tree-cell-text(container, closed, hasUnread, read) {
     text-decoration: none !important;
     font-weight:  bold !important;
     color: #000000 !important;
   }

/*
    * If thread selected, revert to highlight text color
*/
   treechildren::-moz-tree-cell-text(container, closed, hasUnread, read, focus, selected),
   treechildren::-moz-tree-cell-text(container, hasUnread, read, focus, selected) {
     color: HighlightText !important;
   }

• Save the document as filename userChrome.css in the chrome folder. Note the spelling all lower case except for the 'C'.

Double check all the copied code is correct (view info in this forum questions) as sometimes the !important in code part seems to get missed by some people, so not sure if something is being altered in the emails you receive. Edit and resave if necessary.

Start Thunderbird.

If not working then check the following is enabled.

In Thunderbird

• Menu app icon > Preferences
• select 'GeneraL'
• scroll to bottom and click on 'Config Editor' button
• in top search type: legacy
• look for toolkit.legacyUserProfileCustomizations.stylesheets
• If set as 'false' then click on toggle icon on the right to change to 'True'.

Exit and restart Thunderbird.

Awesome! Thank you @Toad-Hall, that last part in the config editor did the trick. I had tried before with just the .css from this prior post but it wasn't working. Plus the actual code in the .css is a bit different from before.

I removed the "color: #000000 !important;" line since it caused the text to remain black in Dark Theme.

Thank you so much for your help. Hopefully this helps others as well.

Regards.