I played with this (on MacOS) a bit lately.
I was unable to accomplish anything useful.
For some reason, I was under the impression that changing the default text color and background color in composition mode would only affect the local display. Looking at the dialog I'm not sure why I thought that; but it definitely affects the sent mail.
I looked for info on thunderbird colors. I found a reference to changing this setting in userprefs.js:
user_pref("browser.display.background_color", "#073642");
user_pref("browser.display.text_color", "#657b83");
This doesn't do anything useful in postbox. It does colorize the empty pane if you have no message displayed though. :)
I found this:
which changes the colors in the message index pane based on message status. I guess if you were a CSS stud and really started poking you could figure out how to do more. If you do please share, and I will buy you a pizza. :)
danno