add CSS core styles

The core styles are used to define build in styles for elements like
processing instructions, that need a stylesheet to be displayed
correctly. The core styles have a lower priority the plugin provided
stylesheets.

Change-Id: I08d85d57e9e10941403d0c1df2a32df60020817a
Signed-off-by: Carsten Hiesserich <carsten.hie@gmail.com>
6 files changed