The site is white in IE9 at my end and seems to miss a lot of styling.
The stylsheet is loaded though, it's visible in the page source.

I do see you have casing issues in your CSS and the skin is not using an XHTML doctype.
(sorry to say but technically it's quite bad, lots of tables etc)
AFAIK, with the doctype your using class casing should not matter, but I still would try this:

Changing this in portal.css:

Body
{
background-color:#333 !Important;
}

TO

body
{
background-color:#333 !Important;
} 

And see if the body background is correct in IE9, if it is check the casing of all your classes.
And change the skins doctype to XHTML if possible (but prepare for some work as that makes all classes and Id's case sensitive)