This can be done by modifying userChrome.css:
userChrome.css in the chrome folder is
a CSS file that can be used to change
the way Mozilla applications'
interfaces look. This file does not
exist in a new profile. You can create
it manually.
From Thunderbird 3.0 - New Features and Changes/Themes:
Color for New Mail
Previously, new messages in a folder
were indicated by an orange star and
the folder turning bold. In the
default theme, the star has been
replaced by the folder name changing
color. Unfortunately, this color is
picked from the desktop theme, and may
result in awkward combinations. To set
the color to something more suitable,
use the following entry:
Windows and Linux:
treechildren::-moz-tree-cell-text(folderNameCol, newMessages-true) {
color: red !important; }
This will change the color for all
states. You can have different colors,
e.g., when the folder is selected
(omit the focus part below), and when
it has focus:
treechildren::-moz-tree-cell-text(folderNameCol, newMessages-true, selected, focus) {
color: orange !important; }
Using the two entries above, the
new-mail indication results in a red
folder name if it isn't focused, and
turns orange if it has focus (in which
case the background color should be
blue with the Windows Classic desktop
theme, thus giving a nice contrast).
You can specify any color as #rrggbb
with 2-digit hexadecimal values 00-FF
for red, green, and blue.