Logo mobil device

Hello, I´m using a full-width (desktop width: 1200px) if I check the site on mobile device the logo will be reduced (responsive) but it look awfull because it´s to small ... Website: nest-electronics.com Now I tried to it replace in customer css #site-logo, #site-header with content: url (http...img.jpg) but it doesn´t work. Can you please help me? Thank you very much in advance.