I Need To Alter CSS Twenty Twelve Theme to Customize View on Mobiles

Hello everyone, can some please help, I am using the Twenty Twelve theme for a one page website. I have the site working both on desktops and mobiles. The Desktop site looks fine but when viewed on a mobile the graphic header could do with moving up a bit and also the space between the header and where the page starts is too big.

Does anyone know how to alter the CSS to fix these issues?
I would be very grateful.

Thank you