This is Dark Theme, that changes the color of the web browser interface! If you want to dark all your websites such as Facebook, Twitter, Google Search, etc. Use the most popular "Turn Off the Lights" Firefox extension. https://www.turnoffthelights.com/firefox.html
A Firefox theme cannot change the color inside the web view container (-> example google.com the website itself). Because Firefox themes can not inject scripts in the web page.
Turn Off the Lights Firefox extension is available for Mobile and Desktop.