Custom styles added to the StyleConfig.js file are not showing up in the editor when you use the Inspector.

A few lines of code are commented out in the 'Workarea\FrameworkUI\js\Ektron\Controls\EktronUI\Editor\Aloha\plugins\ektron\advancedinspector\lib\advancedinspector-plugin.js' file.

To allow custom styling in the editor's Inspector:

1. Edit 'Workarea\FrameworkUI\js\Ektron\Controls\EktronUI\Editor\Aloha\plugins\ektron\advancedinspector\lib\advancedinspector-plugin.js'.
2. Un-comment all instances of: 

   //var thisinstance = new BasicStyle("listid", clickedon, styleConfigJson);    
   //$ektron('.ektron-aloha-advinspector-modal .basic').append(thisinstance.create());  
   

3. Save the file.
4. Clear all cache. Caching for this function can be deep, so you should also clear server, browser, and site cache.