/*
* 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 */

x-current-month[relation="today"],
    .calendar-month-day-box-day-off[relation="today"],
    .calendar-month-day-box-other-month[relation="today"] {
       border: 2px solid red !important;
    }
    .calendar-month-day-box-date-label[relation="today"] {
       color: red !important;
       font-weight: bold !important;
    }

~/.thunderbird/user/chrome/userChrome.css