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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def bootstrap_css():
 """ 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 %} """

    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]))