I'd seen it before: "Invert colors". For some reason, those words never meant anything to me. I'd seen them many, many times. But suddenly I realized what it was for. I enabled it and now I can press a simple combination of keys to turn a white page with black letters into a black page with white letters. And I can toggle back and forth with ease.
Now the screen doesn't look too bright when I visit the NY Times or any other site that uses a white background. And this capability applies to all programs. So when I open a new Word doc at night, for instance, I can make that blinding white page black. Same goes for Mail. Those programs look way too bright to me. The problem was always that if you turned the brighness down, you lost contrast. But this accessibility option makes things crisp and clear -- without the eye pain! I couldn't be happier. This is the solution for my most pressing computer problem.
I'm writing this post on a black page with white text. It's soothing. This may mean nothing to readers but it's a dogsend for me.
PS: You do this through System Preferences/Keyboard/Keyboard Shortcuts/Accessibility. Then check the invert colors box. Voila.