color code messages in thunderbird

In order to color message lines in the message pane in Thunderbird, you need to edit the CSS file "chrome/userChrome.css" in your profile directory. On Mac OS X this is located in "~/Library/Thunderbird/Profiles/xyz.default/". If the CSS file (or the chrome directory ) do not exist, create them.

A possible application is color coding replied messages. The code below uses green color to highlight them:

/*
 * Do not remove the @namespace line -- it's required for correct functioning
 */

/*
 * Set default namespace to XUL
 */
@namespace
url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

/* background for normal message lines */
treechildren::-moz-tree-cell(replied) {
background-color: LightGreen !important;
}
/* background for current message line */
treechildren::-moz-tree-cell(replied,current) {
background-color: Green !important;
}
/* background for selected message lines */
treechildren::-moz-tree-cell(replied,selected) {
background-color: Green !important;
}

More examples can be found here:
http://www.eriwen.com/css/tweaking-thunderbirds-chrome/