There should be a dark theme for mozilla.org and addons.mozilla.org.
The latter is particularly important, since it is a protected page and addons like Dark Reader cannot touch it.
If you're worried about wasting developer time on designing this from scratch, you could cheat by using the Dark Reader npm library. There is an option to export the autogenerated CSS as a string (exportGeneratedCSS). You could use this as a starting point and modify/correct any of the mistakes the Dark Reader algorithms made.
Bonus points if you can make the light/dark themes switch based on prefers-color-scheme so it matches OS/browser theme settings.