Outlook for Android has a pretty good dark mode. If you're using a Samsung phone with Android pie or higher then the Samsung email app should have a dark mode as well which can be set through the system-wide dark mode. If you've got a phone running Android 10, like the pixel 4, then Gmail will default to dark mode if you set the system-wide dark mode


Alternatively you can try enabling the color inversion setting under accessibility in the settings app. This is system-wide. It's not as nice as what you would find on something like zoom text but if you can't find anything good then this should at least work. if you don't want the colors inverted all the time you can enable the accessibility shortcut and set it to toggle color inversion

Fairmail does the job for me since it removes the styling of the mail and leaves it as plain text. It also has dark mode