I just saw that Litmus is offering this in their email tools. Not sure what the current uptake is on dark mode, but with how high iphone usage rates are for emails, would be a great feature to have available.
This would be very handy going forward now that iOS 13 and Android 10 (both released in Sept 2019) contain system wide Dark Modes. Also worth mentioning
Windows 10 (Since 2019) MacOS (Since 2018) Outlook for Mac/Windows (2019) Thunderbird (2019)