I think I'd start by trying a theme. You'll need a so-called "full" or "complete" theme, as many of the available themes just decorate the title bar. https://addons.mozilla.org/en-US/thunderbird/complete-themes/?page=2 However, you do get into some discrepancies. You'll note that most of these leave the message pane untouched, becasue they leave the message as it was sent. Interfering with colours in incoming messages can easily lead to them becoming illegible. You can set foreground and background colours for yourself, but these tend to create problems when you print messages. Ideally you would set these once and for all in your OS and the settings would be applied to all programs. In my limited experience of these things, such adjustments have been withdrawn from Windows (it started with Vista, I think) and in any case, Mozilla products don't seem to pay much attention to global settings. Windows 10 has a "night light" setting that dims the screen and uses warmer colours, following a theory that our circadian patterns are distorted by the blue light from display screens at a time of day when we are expecting the sun to have set. This setting might help you. In Windows 10, '''Start|Settings|Display|Brightness & colour|Night light'''.