If you use a common desktop browser, take a look at an add-on called Color Changer. It allows you to set background, text and link colors independently using a color wheel, it remembers your default on/off settings for each site, and it allows you to turn it on or off with a couple of clicks using an icon in your browser toolbar. It does have a few unintended side effects -- some icons like the notification bell lose their color, and some images in pop-up windows are affected adversely -- but if harsh white backgrounds are bothering your eyes, Color Changer might be worth playing with.
