I did have to adjust my Signature colors some to be able to be seen easier with the dark.Mostly not being able to read blue text on the dark background on some people's signatures. Also, I found it harder to differential between the read and unread threads in the Dark Theme. Even the "New" button didn't stand out as much.
I tried using the dark mode on my MacBook and went back to normal, too. Hard to believe I complained when iOS went from dark foregrounds to light around iOS 7.