Match progressbar to subtler grad we have now

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5380 2fea6a4d-a838-0410-917b-93a53c48e9d2
Provide an alternative progress bar colour option for themers that don't want yellow

They can override jQuery UI's default image within their own theme if they wish

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5381 2fea6a4d-a838-0410-917b-93a53c48e9d2