I assume you are using the Dark theme. You might try adding the addon, Dark Reader.

Thanks for the quick reply David. Unfortunately I'm using the System Theme v1.3, not a real fan of dark themes. I should have mentioned that.

Try the Thunderbird theme choices in the addon manger. While code level settings can be implemented in a userchrome file. It has to be re-coded for each major version release after V115 comes out in a couple of months that is going to be monthly I understand. SO I clearly do not suggest using userchrome to make changes to settings.

I've pored through about a thousand alternate themes and found nothing close to what I'm looking for there. Firstly, I don't want a dark theme, and all the others I've looked at include wild graphics and gradients that only make TB harder to read! I'm looking to increase the readability factor not pretty it up.

I've tried to even find if there's a way to clone the System Theme then edit it to make my own theme but there's no help that I can find anywhere on how this can be done. Guess I'll just have to be content with what I have.

Terry, I was about to suggest some userChrome.css customization, but @Matt's input tells me that has a short life. So, my suggestion for now is to use the color setting options at settings>general. I use that to set background of messages and font color and it works well. It does not influence overall style of Thunderbird, just the messages themselves, and that is my priority -- and appears it might be yours. I don't care for the dark theme, and also dislike a white background, so that setting option serves me well. Good luck.