It requires some mild hacking. Find the while() loop used to display the threads in forumdisplay.php, and add this before it:
$alt_num = 1;
Then, right inside the loop, add something like this:
Code:
$alt_color = ($alt_num % 2 == 0) ? "#FFFFFF" : "#E9E9E9";
$alt_num++;
Then just reference $alt_color instead of "{firstaltcolor}", etc, in your forumdisplay_bit template. I did it with my forums here:
http://forums.thestands.com