The threads, you'd probably have to edit the postbit template itself:
Try changing
Code:
$template_hook[postbit_start]
<table class="tborder" id="post$post[postid]" cellpadding="$stylevar[cellpadding]" cellspacing="$stylevar[cellspacing]" border="0" width="100%" align="center">
To
Code:
$template_hook[postbit_start]
<table class="tborder" id="post$post[postid]" cellpadding="$stylevar[cellpadding]" cellspacing="$stylevar[cellspacing]" border="0" width="100%" align="center" <if condition="$forum[forumid] == 'x'">style="border: 2px solid #ff0000;"</if>>