Rated 4 out of 5 stars
I do really enjoy this plugin. Sadly, it doesn't always work with certain themes on the latest Thunderbird. For example, if you use one of the current dark themes (ie, TTDeepDark), the text turns white, and that doesn't work against a yellow background. Thankfully you can edit it to change it to black (or whatever color you need).
Go to: C:\Users\YOURUSERNAME\AppData\Roaming\Thunderbird\Profiles\YOURPROFILE\extensions
(*change YOURUSERNAME and YOURPROFILE to what you find in those folders)
Extract out "email@example.com\chrome\skin\xnote-window.css"
Edit the section:
Change text color to whatever you need of course.....(use a color hex code website to figure out the 6 digit code if you have to, or you can also replace "#000000" with just "black" or another color (for basic colors).
add it back to the extension package, and start Thunderbird back up. Text color should be changed. :D
I'd give it 5 stars if it had the ability to change the font color in the extension settings or something.