When RadEditor's content area is rendered as Div element, there is no head element in the content (where CSS files to be loaded). Therefore, any custom CSS files that should decorate the content area or to populate Apply Css Classes tool and table's CSS Layout, the custom CSS files should be added directly to the page. 

For example, in this case here, the CSS files passed to the TableLayoutCssFile property should be additionally loaded in the page's head element for the tool to work as expected. 

If you have difficulties about that, do not hesitate to contact us via support ticket.