def render_favicon(context, filename='favicon.ico'):
    """
    Return favicon link for current theme.
    
    By default return the file 'favicon.ico' in root theme dir.
    You can override this file with parameter.
    """
    website = context['request'].website
    if website:
        favicon_link = u'<link rel="shortcut icon" type="image/png" href="%sthemes/%s/%s" />' % (
            context.get('STATIC_URL', ''), website.theme, filename)
        return favicon_link
    return u''