Swarth modifies web pages to use a dark color scheme or based on the user's preferences. This extension is a fork of Dark Background and Light Text, rewritten to utilize XUL overlays instead of Jetpack/Add-ons SDK. Furthermore, several additional features were included, such as following the browser's preferred color scheme and keeping changes made in private browsing sessions.
Simply download the contents of the "src" folder and pack the contents into a .zip file. Then, rename the file to .xpi and drag into the browser.
Check the releases page of this repository.