decided that --themes was silly - no one would do that, just use
    multiple --theme arguments
  made --theme a synonym for --css and -c
  made those arguments, if no file is found, look up the argument as the
    name of a built in theme

  all of this let's haddock be invoked with "--theme=classic" for example.

  The bulk of the change is threadnig the selected theme set through functions
  in Xhtml.hs so that the selected themes can be used when generating the page
  output. There isn't much going on in most of these changes, just passing it
  along. The real work is all done in Themes.hs.