Hello, Lauren, 

After looking further into your requirement, I determined that setting a custom CSS for the content of the HTML editor can be achieved with contentWindow method. This method returns the window object associated with HtmlEditor`s content editable area. Once the area is retrieved, any css of your choice can be set. For example: 

var temp1 = $("#htmlEditor").igHtmlEditor("contentWindow").frameElement;
            temp1.style.backgroundColor="red"; 

Here it is a sample for your reference. Please test it on your side and do not hesitate to contact me if you have any additional questions regarding this matter.