the conditional you need to use is
vB3x
Code:
<if condition="$thread[replycount] == 0">style="background-color:#FF0000;"</if>
vB4x
Code:
<vb:if condition="$thread[replycount] == 0">style="background-color:#FF0000;"</vb:if>
in the
threadbit template. you can place it as a style for the
main <li> element by adding a background color or use as
you wish.
Just guessing, could be wrong. havent tested it, but its a place to start.