In the forumdisplay template
Change this
Code:
<table class="tborder" cellpadding="$stylevar[cellpadding]" cellspacing="$stylevar[cellspacing]" border="0" width="100%" align="center" id="threadslist">
To this:
Code:
<table class="tborder" cellpadding="$stylevar[cellpadding]" cellspacing="$stylevar[cellspacing]" border="0" width="100%" align="center" id="threadslist" <if condition="$foruminfo[forumid] == 'x'">style="border 2px solid #ff0000;"</if>>
In $foruminfo[forumid] == 'x', make sure to change x to the proper forumid of the forum where you want this to be applied to.
Let me know how it works out.