Change validation message for empty CSS file in CSS dialog

1) Open any CSS file
2) Clear it all
3) Open CSS dialog
ASSERT: "Style class is empty" validation message appeared.

I don't have any style classes on a page at all. And this message appears on in the dialog if CSS file is clear. So I propose to change this message to "CSS file is empty" or "No style class selected" or "No style class specified"