[CONTINUUM-2713] use default xhtml style

Remove customised style to make upgrading between struts versions easier.
Note that one customisation was dropped - forms drop the header backgrounds
(th is now td.tdLabel; class=“b” removed from row)

git-svn-id: https://svn.apache.org/repos/asf/continuum/branches/CONTINUUM-2713@1542502 13f79535-47bb-0310-9956-ffa450edef68