Firefox for Android has a reversed mode showing white on black, but only if you first switch the page to reader mode; it's not a general reversal for all displays. This article has more info: View articles in Reader View on Firefox for Android Does that work for you?

Maybe the OP means the dark private browsing theme? https://support.mozilla.org/en-US/kb/mobile-private-browsing-browse-web-without-saving-syncing-info

Ok, that's easy enough. How is reader mode set as default? I'd like in always on and have the opportunity to turn off when needed. Thanks