Hi, 

Yes, you just need to set an event handler for the filterDialogContentsRendered event and using its arguments and jQuery's addClass(). Here's the configuration of the filtering feature that I tried (and it worked like a charm):

{
    name: "Filtering",
    mode: "advanced",
    filterDialogContentsRendered: function(evt, ui) {
        $(".ui-dialog-buttonset > button:eq(1)", ui.dialogElement).addClass("my-class");
    }
}

Let us know if you need assistance with anything else.

Cheers,
Borislav