I like the dark theme but I had to switch back to the light theme because some emails are hard to read with the dark theme.
I believe the viewing pane should default to a white background and not alter the colors of an email, the theme shouldn't change the presentation of an email.
When you create, reply, or forward an email, the editor pane has a white background even with the dark theme. I believe this is the correct implementation, if the email sets the background color show it, otherwise default to white.
Most email programs that have a choice of themes for instance Outlook, the viewing pane defaults to white regardless of the theme.
I'm sure some people like the dark viewing pane, I propose adding a setting for the viewing pane so we can choose.