Perfect Dark Theme geliştiren: Dziad Borowy

To remove the purpleness further:
1. in about:config edit the browser.display.background_color to something like #222222
2. in userContent.css add these:

:host, :root, html, body {
--in-content-page-background: #333 !important;
--in-content-page-color: #eee !important;
}