Diffrenet width for desktop and mobile

[gamestoenjoy]

Hi,

In use WordPress with the theme twenty thirteen.

I have empty spaces when I check my site in desktop, so I want to use width of 50%
In style.css I changed for .site the width to 50% instead of 100% and it worked great in the desktop, but when I checked my site in mobile it also changed the width to 50%
I want the desktop version of my site to be 50% width, but the mobile version to still be 100%

I also added:

.site {
    width: 50%;
}

to all @media besides @media (max-width: 359px) which is used for mobile, but noting happened...
The site is still shown with 100% width in my desktop.

What am I doing wrong?

My site: http://www.convertforfree.com

Thanks for your help,
Oren

[Ripster]

IS there are separate part of the CSS code that is specifically for the mobile design?

[gamestoenjoy]

As far as I know there is no seperate part, but I may be mistaken...
As I wrote the WordPress theme I am using is twenty thirteen, thanks.