The Generic Content module is using the RadEditor for ASP.NET AJAX and that has a drop down for selecting a CSS Class. You can simply click on a word, with that css class applied, and the drop down box will change to the CSS Class. Try it on this online demo
By default, this is turned off in Sitefinity. To turn it on, follow these steps:
1. Open this file: ~\Sitefinity\Admin\ControlTemplates\EditorToolsFile.xml
2. Add this line of code inbetween any of the <tools> tags:
<tool name="ApplyClass" />
The RadEditor has an HTML view too, which you can get by clicking on <>.