How to dim websides

When working for hours on the PC the eyes are getting tired and it's very nice to dim the side to a dark background with bright letters.
It's far less demanding for the eyeseight,as allways looking at a bright screen.

Firefox offers a nice AddOn for this purpose:Blackout

Here the blog in black:

Blackout can be activated by clicking on the bulb next to the Google search bar.This way is also a menue available,where the color can be choosed or to "stay in the dark"permanently.