Browser compatibility with IE?

Please excuse the fact that I’m completely ignorant of CSS for the most part.

I modified the default theme and was finally happy with the sidebar when I realized that it wasn’t compatible with IE (I haven’t checked other browsers yet). I remember having a similar problem with a theme I edited.. well over a year ago at this point, probably…

Is there any way to make it so it looks like this while still having it compatible in IE?

http://fantasyislove.com/robotsinagrid/