This is down to your CSS and how it interacts with code affecting the positioning of the post date
[span class="post-date float-left"]
and post menu
[div class="classic-right-menu shaded-bg-color weak-color"].
No doubt there would be further issues in threaded and list views and perhaps elsewhere where the styling is inherited from your [body] styling!
The simplest way to achieve your objective, of limiting the forum width and centring it would be to embed the forum in a page that contains the styling code you quoted.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.