I use Thunderbird as my mail client in many cases. One of the things it does that I don't like is highlight folders with new mail with the color red. To change this behavior, create a userChrome.css file in %AppData%\Thunderbird\Profiles\<your profile>\chrome with the following information:
/*
* Do not remove the @namespace line -- it's required for correct functioning
*/
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
/* set default namespace to XUL */
#folderTree > treechildren:-moz-tree-cell-text(isServer-true, biffState-NewMail)
{
font-weight: bold !important;
color: #3357DD !important;
}
#folderTree > treechildren::-moz-tree-cell-text(folderNameCol, newMessages-true)
{
font-weight: bold !important;
color: #3357DD !important;
}