[imported_DeSantes]

Well, after a bit of Google research I figured it out. I used amykhar's alternating colors code as a base for my research;

<if condition="$forum['displayorder'] % 2 == 0">class="alt2"<else />class="alt1"</if>

First I had to figure out what the hell the % sign was. Here's the explanation from a Google find;

While you are no doubt familiar with the arithmetic operations of addition, subtraction, multiplication, and division, the remainder operation may be unfamiliar to you. The remainder operator '%' takes two numbers and determines the remainder after dividing the first by the second. For example, consider the expression '11 % 4'. Since 4 goes into 11 twice with a reminder of three, '11 % 4' evaluates to 3. Similarly, '10 % 2' evaluates to 0 since there is no remainder after dividing 10 by 2. If this operator still seems a little odd to you, try out several more expressions in the JavaScript interpreter.

The remainder operator can be quite useful in determining certain properties of numbers. For example, consider the expression "X % 2" where X is some arbitrary integer. If X is even, then the remainder after dividing X by 2 will be 0. If X is odd, then the remainder will be 1. This expression, then, provides a simple test for determining whether an arbitrary integer is even or odd.

Now, since we can't use 'displayorder' to check for odd or even numbers, we'll use the 'threadid' instead.

Put the following code in your threadbit template in place of the CSS class calls;

<if condition="$forum['threadid'] % 2 == 0">class="alt2"<else />class="alt1"</if>

Works for me, I hope it work for you.