User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:70.0) Gecko/20100101 Firefox/70.0
Steps to reproduce:
Set the OS color theme to dark (Ubuntu Mono Dark) and Thunderbird to light theme.
This only happened after I upgraded to Ubuntu 19.10 (and presumably Thunderbird 68.1.2 at the same time).
Parts of the UI are dark (e.g. the message thread overview, scroll bars, the selected day in the calendar sidebar list) or inconsistent (highlighting is sometimes orange and sometimes blue). I can't even see the cursor in the main scrollbar.
In Thunderbird's dark mode on the other hand the text colors are not suitable. Text is unpleasantly light and the tag colors don't go well with a black background.
Everything should consistently match the chosen theme and colors should go together well, independent of the OS theme.