You didn't bother to search.
Feeder, FeedDeck, NewsBlur, Fluent Reader, Twine, Capy Reader, Reeder, Read You, Neo Feed, Readrop are all open-source RSS/feed readers for mobile (iOS, Android, or both) , and they all have dark theme / night mode.
https://alternativeto.net/software/feedly/?license=opensourc...
https://duckduckgo.com/?q=android+ios+open-source+rss+reader...
I don’t rate search “knowledge” as valid in this context. You clearly don’t use these or experience things in the same way.