How can I apply css styling to the current day in the calendar widget

On my default calendar widget I get:

<td id="today">30</td>

so you could use #today.

It uses the get_calendar() function where we have the following:

if ( $day == gmdate( 'j', $ts ) &&
    $thismonth == gmdate( 'm', $ts ) &&
    $thisyear == gmdate( 'Y', $ts ) ) {
    $calendar_output .= '<td id="today">';
 } else {
    $calendar_output .= '<td>';
 }