Selected text background color
To change the background color of selected text, exit Thunderbird and add the following to your user.js file, replacing "#FF0000" with the hex value of your own preferred color. This seems to affect text selected with the cursor anywhere in Thunderbird, such as in the message preview pane and in dialog boxes.
// Colors for selected text: user_pref("ui.textSelectBackground", "#FF0000");
If you want to change both the background and foreground colors of selected text, use the following instead:
// Colors for selected text: user_pref("ui.textSelectBackground", "#FF0000"); user_pref("ui.textSelectForeground", "#FFFF00");
For other colors, this Color Blender is convenient. Note that some color combinations may behave erratically, in some cases with the background and foreground colors getting transposed. Initial testing suggests that better results are obtained if you use a darker background color rather than a light one.