I use a dark theme in Ubuntu. Everything is fine in Thunderbird except for the messages that arrive where a font color is specified ( I assume Outlook forces a color to be specified). Those messages of course use the font color specified in the incoming message. This is a problem for my dark theme as the font color specified is typically a dark blue (difficult) or black (invisible against the theme's black background). How can I force Thunderbird to use my preferred text color and to ignore the text color specified in incoming messages?
I had exactly the same problem. I fixed it by choosing View->Message Body as->Simple HTML. It's the best I can do. Just read carefully the right section on the link provided by Oliver Salzburg on the comments.