Surpress margin on mobile device

Hi,
I added some CSS code (margin-right) to move the menu bar at the top of the home page so it would sit over to the right hand side of the screen

.prime-inner {
margin-right: -300px;
padding: 5px;
position: relative;
float: none;
}

But when I view the page as responsive using firefox the menu bar is too long for the screen and there is a scroll bar at the bottom. I cannot view it on a mobile device as the website is offline, but I can view a responsive mode using the firefox development tools. I suspect it will look the same on a mobile with the scroll bar at the bottom. Is there a way to disable the margin-right line from being used on a mobile, or enter another line of code for mobile. If I delete that line, the site displays properly on the mobile, but the menu displayed on a computer screen is moved over to the centre.

The page I need help with: [log in to see the link]