In the administrator CP, edit the threadbit template, about in the 1/3 of the code you'll see this:
<if condition="$show['gotonewpost']">
<strong><a href="showthread.php?$session[sessionurl]t=$thread[threadid]$thread[highlight]" id="thread_title_$thread[realthreadid]">$thread[threadtitle]</a></strong>
<else />
<a href="showthread.php?$session[sessionurl]t=$thread[threadid]$thread[highlight]" id="thread_title_$thread[realthreadid]">$thread[threadtitle]</a>
</if>
Change it to this:
<strong><a href="showthread.php?$session[sessionurl]t=$thread[threadid]$thread[highlight]" id="thread_title_$thread[realthreadid]">$thread[threadtitle]</a></strong>
As you may understand though, it'll be harder to tell in which threads there are new posts, since all of them will be bold.
If you also want to make the table prefixes (e.g. "sticky" or "moved") bold, find this, it's just over the code above:
$thread[movedprefix]
$thread[typeprefix]
$thread[moderatedprefix]
And change it to:
<strong>
$thread[movedprefix]
$thread[typeprefix]
$thread[moderatedprefix]
</strong>