First of all thank you for the great job done. TB 78 has finally come out and represents a big step forward for all of us.
With TB 79.0beta 1 (but I had the same problem with any version of TB 70.0+), in Windows 10 in dark theme mode and with TB both with Dark theme and with Default theme (theme with OS color scheme) the emails that arent send from TB are less readable.
The same email in the gmail mobile app (I don't know if the comparison is adequate) are perfectly legible. It seems to me that the difference lies in the fact that gmail overwrites the forecolor of the text of the HTML message, while TB does not, it leaves it as it is. I am attaching a screenhot of the issue-in-action in TB 79.0 beta1 and the same email in the gmail mobile app.
Ciao.

Same issue here. Have been able to mess with the settings a little bit to get some html content to display better but still not correctly when using dark theme in Windows 10.