I prefer a dark theme. I've lived with thunderbird doing a bad job of that for a while.
I finally decided to fix it. I found this
page which had what I wanted.
To change the color of the links in email messages, edit userContent.css and change the color for a:link. I set the background and foreground color also (they are set in the theme also, but this seems to help sometimes.
body {
color: white;
background-color: black;
}
a:link {color: #a53f10}
td {color: black; background-color: white}
Then to change the color of the links for From: To:, edit userChrome.css to change the colour of the links there.
.emailDisplayButton {
color: #a53f10 !important;
}
These are light orange colour, change to your preference. They should be pretty obvious at least (you can see what you changed)...