That's pretty easy... in postlist.php, find
if (($d == $thisDay) && ($mon == $thisMon)) {
echo " class="darktable"><table width="100%" cellpadding="0" cellspacing="0"><tr><td>$d.</td>";
This prints the starting point for today's cell. It shows up in "darktable" colors, and other days show up in "lighttable" colors. You can either modify these in the stylesheet you use, or you can modify the above lines to your liking. The $d prints today's day of the month, so you could replace it with <font=red>$d.</font> or something like that.
Regards,
Max