What you are referring to is a 'title' element on template 'threadbit'. Title elements can not be styled directly therefore there is no CSS for this element.
Code:
<div class="threadinfo" title="{vb:raw thread.preview}">
What can be done is to use javascript/jquery for a tooltip and unlink this one from the title element. vBulletin might already have javascript hooked into its title elements, but I am not seeing one being fired off. Still I could be missing something.
I know this doesn't help, but at least you can stop searching for the CSS!
If you google javascript tooltip you will get lots of information. I would use a jquery solution if you are a beginner.