Unfortunately, it isn't that easy. The same template routine (PageTop in public_forum_summary) is used for both category-only mode and normal mode.
So, the change is going to be a little more complex.
First, remove the $TBT from PageTop, around line 55.
Then go find sub Cat_Table_Top, and just insert the modified TBT value in the HTML that it returns.
Finally, go find sub Normal_Table_Top, and insert a $TBT in the very first HTML block (my $Top = qq~...)