The portions of the CSS that fail the validator I suspect are the lines dealing with opacity in IE, such as:
in modal.css, and:
in dhtmlwindow.css. From W3C:
One thing you can do is remove the non standard lines in your CSS that concerns IE, and use conditional HTML to output them:
The CSS Validator validates your style sheet against some profile, e.g. CSS Level 1 or CSS Level 2. Some browser Vendors however extend these profiles by new properties like Microsoft did with the scrollbar-base-color or filter property or Mozilla with the -moz-opacity property. The Validator is not aware of these properties and using them makes your style sheet invalid in terms of the CSS specifications. Either get rid of those properties or live with the fact, that your style sheets don't validate. You cannot have both.
This should take care of IE, though there are also lines used in the CSS that are Firefox only, such as
-moz-opacity: 1;, so you may need to deal with those some other way.