sorry, what is the issue? you want the forum centered, but not be full width of the page? better match the width of the page elements above it?
first, try going to forum - integration - page and permalink and turning on the strict wp api option and see if that helps... you might also want to set display in loop to true and display multiple to false on that panel...
if not, this appears easily corrected by adjusting the forum css... the wp theme is giving the page content (forum) unlimited width (not constraining it to the page)...
If you create a sp child theme (https://simple-press.com/docum.....ild-theme/) then in the child theme stylesheet, add this css:
#spMainContainer {
margin: 0 auto;
width: 1080px;
}
and you should be good... note there will be bunch of space above the forum, but that belongs to the wp theme and the page header... you could remove it with 100 or 150px of negative margin in the above CSS...
if there is something else you want to change, please explain further...