I found this thread after a quick search of the forum. First, great app, Aqua Mail is really well executed.
Regarding the thread, I see the same thing with toast messages on my Pixel 2 (non XL) running Oreo 8.1. I believe with the light theme the font in the toast messages is OK, but in the dark theme the font becomes the same color as the toast message background and cannot be read. I have turned off toast messages for now, but I was wondering if there is a setting to set the font color of the toast message that I am missing?
Much thanks