Hello Greg,
Thanks for asking. 
Looking at the screenshot it seems that the CSS is not getting inherited or the CSS file is not present on the server and that’s why layout is getting broken. I suggest you to try installing the WpForo plugin and see how it goes. To do that please deactivate the plugin from Plugins > Installed Plugins and then install the same.
You can also try adding the shortcode in Classic Raw Content Element and see how it goes. However, issue revolves around a 3rd party plugin and providing support for 3rd party plugin falls outside the scope of support we can offer.
Thanks.