How about View|Folders|Unread? That will at least reduce the amount of stuff you have to work through. Probably not useful if you aren't moving messages to folders by means of filters.

Or the QuickFilter, which has an "Unread" filter option. See the attached image.

Or you could create a Saved Search folder for unread messages.

You can set up code to tweak the colours but it's rather techie and not trivial to do.

I rather go the Techie route if someone tells me what to add or change. Habitually I would not use an Unread filter. But thanks for the ideal.

Saved Searches are easier to set up. ;-)

I'm looking for how to change the underline to a colour. Trouble is, the underlining behaviour is native and so doesn't appear in sample css files.

Here is an old thread, but you may be able to make use of it.

http://forums.mozillazine.org/viewtopic.php?f=28&t=345979

This seems to work:

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

/*
    * Display unread messages in bold black.
    */
   treechildren::-moz-tree-cell-text(unread) {
     font-weight: bold !important;
     color: #000000 !important;
   }

   /*
    * If this unread messages is selected, revert to highlight text color
    */
   treechildren::-moz-tree-cell-text(unread, focus, selected) {
     color: HighlightText !important;
   }

   /*
    * On a collapsed thread, if the top level message is read, but the thread
    * has unread children, set the 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 this collapsed or expanded thread is 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;
   }

You can of course tweak to suit.

I find that the order in which these statements are used is significant. If you already have a userChrome.css, place this stuff early, just after the gatekeeper line, which must be the first active line in the file.

If you don't have it already, you need to create a folder named chrome in your profile. Inside that you put the above text, in a file named userChrome.css. Note that your profile will be hidden by default, so you'll need to enable viewing of hidden files, and preferably, viewing of filename extensions.

thank you Zenos. I am on a Mac. So I created the chrome folder in my profile folder and created the file and copied what you suggested and still all I get is an underline. Here is the path to the file myusername/Library/Thunderbird/Profiles/q0zf5vao.default/chrome/userChrome.css I even changes the color from #000000 to #8A2BE2 but no change.

OK I found out the issue is so stupid. I copied the text of the css code from the email I received in TB. In TB the "!important;" was dropped completely.

For example this line: font-weight: bold !important; shows in TB update email as: font-weight: bold ;

Now I copied your CSS from the support page and it is all well. Thanks

Zenos solution works well with threads in this location, under Linux. I find it much more visible than defaut underlining and straighter than clicking "Display unread only". Thank you nashmarkt and zenos.