User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:89.0) Gecko/20100101 Firefox/89.0
Steps to reproduce:
I am on Linux (Pop!_OS) using dark theme. I have Thunderbird (78.11.0) built in dark theme enabled.
Some HTML based messages displayed in the reader pane retain a white background but the text displays in light grey and is effectively unreadable. It appears that the background colour in the email is following HTML specification (? perhaps?) but the text is changing to match the dark theme.
This can be overridden by setting:
Preferences->General->Language and Appearance->Colors...
with "Text and Background" set to black text and white background, and "Override the colors..." set to always.
This makes all the messages legible, but loses intended colouration (e.g. colour highlights in text).
Image attached ilustrating the issue. The dark text was composed in thunderbird and was included as a reply in the problem message. I can supply the original email if required .
When a message is displayed in its "natural" HTML, the text should default to black unless otherwise specified and not be overridden by the display theme.