This is a userstyle (or theme) to turn the Google Search website dark, also often described as 'Night Mode'.
The aim is to be highly customizable and extremely comprehensive.
It is currently in beta, meaning customization has not been throroughly tested yet and some more obscure widgets might not be styled yet. If you notice something along those lines, please take a look at the issues and open a new one if your problem isn't already reported.
To use it, first install the browser extension Stylus (not Stylish) and then click here.
The mobile site is currently not supported.
(There is currently a bug in Google Chrome that prevents some icons from being colored correctly.)