Since my eyes have been quite sensitive to light recently and gives me a headache if I stare at the screen all day, I made a few changes from reducing the monitor brightness, replacing my desktop wallpaper with a dark themed one, to installing FT DeepDark skin for my main browser, Firefox. I also tried to use Black background and white text add-on to kill the bright background of most web pages.
Black background and white text provides 4 rendering methods in changing page colors: simple CSS, JS mutation, invert, and stylesheet generator.
This restartless add-on changes color automatically as you open a website. It uses simple CSS as the default method because it is the fastest, but you can choose another one via its Options in the Add-On Manager.
Read also: Toolbar Button for Firefox Add-on Options
It adds a button to the main toolbar from which you can switch between the four methods. Select one of them and your current page will be reloaded automatically to apply the color changes.
One of the features that I like most from this one is that it can work globally, on a per site and per web-page basis, so I can switch to another method when it turns out that the default one doesn’t work well on some pages. Also, the changes remain in effect the next time I revisit the site. Another simple but helpful option is that it can be disabled, for example, when you open a website that has already a dark background.
You can install it from here: https://addons.mozilla.org/en-US/firefox/addon/black-background-white-text/
Another option is to use a simpler solution like Page Colors and Fonts Buttons. It simply adds two buttons on the main toolbar to toggle colors and font types. It lets you use your own color scheme but there is no option to save it on a per site basis and the colors may hide some icons and buttons.