Dark Theme Fonts Too Bright
The font used to display the name of the sender and the text of the message are too bright in the dark theme, ironically making it more difficult to read.
Since the background of a Teams channel is the darkest background in the app, the text needs to be dimmer to make it easier on the eyes.
If you compare to the fonts used in the Teams and Channels list/explorer, they are the perfect contrast to the background in that part of the app.
Looking at the Discord colour scheme, the server list background is the darkest, the channel list lighter and the message area is the lightest. I found with my development environment that having a grey background is easier on the eyes than a black one. I think this is the solution
Sean Ellis commented
I see also that other people have complained that the dark theme is too dark. This just underlines the absolute necessity to give people meaningful customization options for their UI.