/*In threads view - collapsed thread - top level needs to be bold if thread contains unread mail.*/

#threadTree tbody .children.collapsed[data-properties~="hasUnread"]
  :where(td, .subject-line) {
  text-decoration: none !important;
  font-weight: bold !important;
  color:  var(--thread-pane-unread-color) !important;
}

I use a 'userChrome.css' file to use 'bold' if collapsed thread contains unread mail. Not sure if you already use a 'userChrome.css' file or not, so I'm including full instructions assuming you have never created a 'userChrome.css' file. First - check in Thunderbird to see if a setting needs switching on. * Settings > General * Scroll to bottom and click on 'Config Editor' button * In search type : '''legacy''' * look for this line: '''toolkit.legacyUserProfileCustomizations.stylesheets''' * If it is set as 'false' : * click on the far right arrow type icon to toggle from 'false' to say ''''TRUE'''' Now Do this: In Thunderbird * Help > Troubleshooting Information * Under 'Application Basics' - half way down is Profile Folders - click on 'Open Folder' A new window opens showing the contents of your profile name folder. * Exit Thunderbird now. This is important. * In the 'profile name ' folder create a new folder and call it ''''chrome''''. The spelling is important - note it is all lower case. See attached image as a guide. * Then open a simple text editor program such as 'Notepad'. * Copy all the text between the lines below and paste it into 'Notepad'. ------------------------------------------------------------------------------ <pre> /*In threads view - collapsed thread - top level needs to be bold if thread contains unread mail.*/ #threadTree tbody .children.collapsed[data-properties~="hasUnread"] :where(td, .subject-line) { text-decoration: none !important; font-weight: bold !important; color: var(--thread-pane-unread-color) !important; } </pre> -------------------------------------------------------------------------------- * Save the file as ''''userChrome.css'''' - note the spelling of this is important - all lower case except the C. * Save it in the ''''chrome'''' folder. - see attached image as guide. Check that file really is called 'userChrome.css' and not 'userChrome.css.txt' - it has to be saved as a 'Cascading Style Sheet (css)' and not a text document. In image below you can see the 'Type' column says it's 'Cascading Style Sheet (css)' and not a 'Text document'. So if you see file has somehow been saved as 'userChrome.css.txt' - no problem - just rename it by removing the '''.txt''' - you will get ask are you sure as it may effect things - just say yes because you really do want to do it. Start Thunderbird

Thank you very much for your prompt response! it works!