Dialogue box exit "X" icon not appearing.

Here what I see in your custom css:

This is the css file which loads all custom CSS of wpForo. you should scroll down and see the custom CSS on the bottom of all code:

https://the-great-work.org/wp-content/plugins/wpforo/wpf-themes/classic/colors.css?ver=1.4.13

As you can see the CSS code we provided is not there. Please navigate to Dashboard > Forums > Settings > Styles admin page, put the following CSS in "Custom CSS Code". Save it, delete all caches and do Ctrl+F5 on front-end. OR out it in WordPress Theme Custom CSS area.