Install

First install a userstyle manager :

Stylus for Firefox
Stylus for Chrome/Chromium
FreeStyler for Safari

These extensions are the best for me, but you can use any userstyle manager.

With the repo

Then go to the style that you want and click Raw.
Then, click "Install style" (or similar depending of your userstyle manager).

With userstyles.org

Find the styles here (they may be outdated).
Select one then click "Install Style" to install it.