There's a Dark Reader extension that will do that for you, if you have fdroid I'd also recommend grabbing Fennec instead which deblobs Firefox, changes some bad defaults, and enables about:config

One option is to create an rss feed of a subredddit and browsing it with a reader of your choice

How to do it: https://www.howtogeek.com/320264/how-to-get-an-rss-feed-for-any-subreddit/

Options for rss readers: Fluent Reader on desktop, Feeder on Android, and NetNewsWire on iOS