How to make website content or whole theme responsive while creating website on Themler?

Hi I am creating a new template for an existing Wordpress website below. http://mustangequinetransport.co.uk I have created a duplicate of the site above which is located below and activated the...

I wonder how to change the theme for mobile-devices for instance. My theme looks good on computer but not so good on mobile phone. How can I change the theme for only the mobile? Site:...