Add watermark.

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')