Not just for 1 website. We need for every firefox webpage including extention store. We're currently relying on 3rd party add-on to do the job whereas other other browser have native dark mode for webpages.
it would still be better to have a native dark mode in Firefox. I am using an extension "dark mode" and moving the switch up and down can get all pages dark, including this site, but it takes a few extra steps each time to do this.