Isso pode ser feito modificando 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.
De Thunderbird 3.0 - Novos recursos e alterações / temas :
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 In the default theme, the star has been replaced by the folder name changing color.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.