This blog is just to make people aware of a small issue (I would call it a small issue) with the integrated CSS Editor in Design Studio 1.6.
Lets assume you are starting a new application in Design Studio and you navigate to the Custom CSS property for the application.
You then start the integrated CSS editor
.. and you might notice that the file extension is missing already
... and when you save the CSS, the extension is also missing in the property.
Basically - unless you add the CSS Extension manually to the file - it seems that the integrated CSS editor is not adding the file extension automatically, which means that your CSS is not working.