Not sure if I'm posting this in the correct place ie is it a 'Dark mode' interest or an 'email' thing, so moderators please feel free to move it if needed.
In response to a query I made about 'When' Readdle would develop Dark mode for Spark on iOS / iPadOS, I was very impressed with their quick response, indicating that A / it would be 'really soon' and B/ offering me Beta version software in Dark mode to report back on. I'm usually very hesitant to use 'Beta anything' as I don't have the technical savvy to untie knots should they happen, but I have always been impressed with Readdle, so went ahead.
The download / install were very simple ( from a link in their reply letter) and it's now up & running on iPhone & iPad 13.1.2. Seems great and simply a Dark mode version of what I've used for a few years. I did the Dark mode on my iMac last year, when I first started with Dark email but have not as yet done Catalina - it's in 'steep like tea' stage as I await bugs if any to fly out of the carpet, probably until mid November +/-