You using this property in the wrong way.

This setting use to  change the editor content style when edting them.  and it apply for the html target or with or without the class setting. it alos use to custom the css class dropdown list of the editor, more details please refer to http://cutesoft.net/developer+guide/scr/Class_dropdown_Customization.htm

for example, if you have the style setting below in the stylesheet, then you can use <div class="myColor">content</div> in the editor to make it use color blue. this is how the html class property work.

.myColor

    {

         background-color: blue; 

}

Regards,

Ken