Just tested this, seems to work OK.
In your threadbit template, look for this (first thing in the template!):
Replace it with this:
Code:
<tr style="background-color:<if condition="$thread[statusicon] == '_dot'">#fff;</if><if condition="$thread[statusicon] == '_hot'">#f00;</if><if condition="$thread[statusicon] == '_lock'">#000;</if>">
Then every time you seen class="alt1" or class="alt2", remove it. That means that when you have a replied to a thread the row is white, when it's a hot thread it's red, when it's locked it's black. Change the colours to what you want, obviously. The reason it gets nasty is because there's lots of combinations as well: _dot_hot is when a thread has been replied to by you and is locked. You will have to add a conditional for each combination or it will be white by default.