Antora: Update UI style/theme for local preview
    
    We are using an old version of the Fedora Docs theme and UI package.
    This commit updates the local Antora config used for the local preview,
    so that anyone working on the Fedora Council docs would also see the
    same preview locally that they would expect on the production site.
    
    Signed-off-by: Justin W. Flory (he/him) <jwf@redhat.com>