Responsive Twenty twelve

Hi,
I’m using Twenty twelve to make a responsive site. I added a div in the head section and I have difficulties to adjust it to explorer 8. This version of explorer does not read responsive code and I want to add an other background image to the head section. I can’t get it to work. I have a css-class for the section in the IE css-document but the styles just been overridden by the original css-document, why?

Thank’s for help!

This is the IE css-code:

body .tophead{
max-width:1020px;
background:url('http://www.mysite.se/wp-content/uploads/2013/03/head.png') no-repeat top left;
padding:30px 15px 30px 15px;
}

This is the original css-code:

body .tophead{
margin:20px auto 0 auto;
margin:1.428571428rem auto 0 auto;
max-width:1010px;
max-width:72.142857142rem;
max-height:100px;
max-height:7.142857142rem;
min-height:52px;
min-height:3.714285714rem;
background:url('http://www.mysite.se/wp-content/uploads/2013/03/head_mini_ver2.jpg') no-repeat center left #fff;
}