Css Style will be overwritten?

Hello

I have a CSS style for my table to format the text in header left:

#ctdata th { 
    text-align: left;
    background:#6CF;
}

The table is created like this:

oTable = $("#ctdata").dataTable({
            "sAjaxSource": ajaxSourcePhp,
            "sAjaxDataProp": "",
            "autoWidth": false,
            "aaSorting": [[ 0, "asc" ]],
            "scrollY":        "200px",
                   "scrollCollapse": true,
                   "paging":         false,
            
            "pagingType": "simple",       
            "bDestroy": true,
            "language": {
                "decimal": ",",
                "thousands": "."
            },

When the page is loaded the css style will be overwritten.
Heading of table is always middle and white background.

Why this?

thanks
regards
Hawk