Based on the AjaxControlToolkit API for the CalendarExtender I can control the style of the selected date using:
.ajax__calendar_active { background-color:red }
And according to the API, I was hoping, that .ajax__calendar_today would allow me to control the style of today's table cell, if show. Unfortunately, this .ajax__calendar_today controls the style of the bigger "Today: XYZ" button at the bottom of the calendar.
Does anyone know how to style today's table cell, if/when displayed?