Load the css_gen.css file first and then load the css_IE.css file.
<link rel="stylesheet" type="text/css" href="https://stackoverflow.com/css_gen.css">
<!--[if IE 6]>
<link rel="stylesheet" type="text/css" href="http://stackoverflow.com/css_IE.css">
<![endif]-->
Hope that solves your problem.
And try this too http://css-tricks.com/how-to-create-an-ie-only-stylesheet/
solved Browser specific css for Internet Explorer [duplicate]