Wouldn't that be done better as a global switch, applied by the OS rather than in separate applications? Maybe you should be arguing this with Microsoft, Apple or display hardware manufacturers. My admittedly brief read-up on this topic (I'm always wary of what might be pseudo science) suggests that removal of blue light is more relevant than inverting the colours. For myself, I have sight issues that make reading white characters on a black ground somewhat tiring, so I'd be looking for a dimming rather than an inversion. You could of course, in Thunderbird, choose a theme that offered a dark background and set a suitable colour for the characters. Bear in mind that a traditional LCD backlit screen will still be generating a broad spectrum of light, so there will still be a blue component, only somewhat attenuated.