code: def bootstrap_css():
      
      rendered_urls = [render_link_tag(bootstrap_css_url())]
      if bootstrap_theme_url():
          rendered_urls.append(render_link_tag(bootstrap_theme_url()))
      return mark_safe("".join([url for url in rendered_urls]))
docs: Return HTML for Bootstrap CSS.
      Adjust url in settings. If no url is returned, we don't want this statement
      to return any HTML.
      This is intended behavior.
  
      Default value: ``None``
  
      This value is configurable, see Settings section
  
      **Tag name**::
  
          bootstrap_css
  
      **Usage**::
  
          {% bootstrap_css %}
  
      **Example**::
  
          {% bootstrap_css %}