XWIKI-11190: The design of DW is broken during an upgrade between 5.4.5 to 6.2.1.

* Adding the skin to the URL of the default CSS.

(cherry picked from commit 040bfc37f8258467adaf615e4c33144c10dbbee9)
XWIKI-11190: The design of DW is broken during an upgrade between 5.4.5 to 6.2.1.

* Adding a comment.

(cherry picked from commit c5607c6bbdcfc27007772dcd265652af688ad9a2)