Follow up to r1828588.

Update css to have permalink more in line with httpd.a.o:
   - default to white (blue on dark-blue is not really visible)
   - visible only when the mouse is over the corresponding heading

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1851167 13f79535-47bb-0310-9956-ffa450edef68