Solution:
In /templates/j51_london/inc/style.php
line 301: custom_sw replaced with customcss_sw
// Custom.css
$document->addStyleDeclaration($this->params->get('custom_css'));
if($this->params->get('customcss_sw') == "1") {
$document->addStyleSheet('templates/'.$this->template.'/css/custom.css');
}