so we need to work out which theme template is rendering the forum pages
looking at the page you gave me, I see that the template has a div of
<div id="content-area" class="clearfix">
This is probably a common div to several files, but we can use it’s presence to track down the file
so you need to look in your theme (wp-content/themes/%your-theme-name%) for likely files
If you have notepad++ (or another text editor that will do this) you can download your theme to your computer and do a search across all the files.
otherwise you might have files called page.php, single.php, content.php, content-page.php etc.
in each file where you find the <div id="content-area" then add a further class, but call each one something different
eg
<div id="content-area myclass1" class="clearfix">
<div id="content-area myclass2" class="clearfix">
etc.
then use the browser inspector and hopefully you’ll find the class repeated in there which will tell you what template it is using
then come back