No solution, but a comment: when 115 rolls out, current techniques for color may be obsolete, as my own usercontrol file, which had many color settings, does not work with the beta. So, even if you get the solution you seek, it may not work in near future.

The method given here still works in TB 102, but it will need to be modified for the next release, TB 115.

Help/More Troubleshooting, Profile Folder, Open Folder, close TB, create a new folder named chrome, create a new document in chrome with a text editor, name it userChrome.css, Save as type: All files *.*, copy in the above code, change the colours and numbers as desired. Double-click toolkit.legacyUserProfileCustomizations.stylesheets to true in Settings/General/Config. editor, restart TB.

Online css generator

This is the easiest and simplest method to achieve the result.

Thanks to david and sfhowes for your replies. I know my request is not a big deal, mainly aesthetics/appearance but it would be nice if the header row could be noticeably different from the message list in some way (spacing, font, color or other). Is there some way this request can be shared with the Mozilla team as something to work on for future updates? I'm no programming expert, but I would think it would be a simple tweak to the existing program.

Thanks to david and sfhowes for your replies. I know my request is not a big deal, mainly aesthetics/appearance but it would be nice if the header row could be noticeably different from the message list in some way (spacing, font, color or other). Is there some way this request can be shared with the Mozilla team as something to work on for future updates?

Try using the feedback link one the help menu. It will take you here. https://connect.mozilla.org