docs: def watermark(app, env):
      """Add watermark."""
      if app.config.sphinxmark_enable is True:
          LOG.info('adding watermark...', nonl=True)
          buildpath, imagefile = getimage(app)
          cssname = buildcss(app, buildpath, imagefile)
          app.add_css_file(cssname)
          LOG.info(' done')