We tried to replicate how Windows Mail treats the dark theming of messages. When you use dark mode we do provide a "Light" you can turn on next to the email. This gives you an easy toggle to view the email in it's original format with no content modifications.
There are many many edge cases that our "dark-themify" code may not handle. If you would like you can contact support and send a copy of some of those emails and we can try to make sure we handle those cases. There are certain things we can tweak like the difference in contrast between background and foreground elements and having examples adds to our database of tests that we click through when verifying that we are correctly dark-theming messages.
EDIT: Not saving the state of this "Light" is by design since we wanted people to use in on a per email basis. With our hope being that in most emails it should not need to be used. Still open to potentially changing this behavior, just mentioning that it was our original intentions.
Matt Petty
Senior Software Developer
SmarterTools Inc.
www.smartertools.com