This could probably be done with a template edit... You'd need to know what variable the number of thanks is stored in so that is probably dependent on which thanks hack you are using. Let's assume it's $post['thanks'] - I'm just guessing because I don't use a thanks hack...
But if it was that you could edit your postbit/postbit_legacy template...
find the code:
Code:
<div class="postbody">
Replace it with:
Code:
<div class="postbody" <vb:if condition="$post['thanks'] > 10">style="background-color:yellow;"</vb:if>>
That would change the background to yellow if there were more than 10 thanks.