On windows 10, if the font has been specified in CSS with 'sans-serif',
the insertion cursor on an empty entry widget is not shown. Choosing a
different font such as 'arial' is OK. As reporter based there changes
on the example CSS file remove the reference to 'sans-serif'
With the requirement for Gtk+ 3.22, the version checks dealing with CSS
can be removed and the application CSS files dealing with version 3.10
can be removed with the remaining being renamed.