I've just experienced the same problem

when digging deeper I've figured out that style rf-edt-tbl-hdr defines background-image property which cannot be overriden

my style:

.my-edt-tbl-hdr {

    background-image: none;

    background-color: #FBFBFF;

}

extendedDataTable.ecss:

*.rf-edt-hdr, *.rf-edt-tbl-hdr {

background-image: url(/admin/ui/rfRes/colHdrGrad.png?v=4.2.0.Final&db=eAFjZJBjZDBiZBBh!P!myz!G!9ee!WZgAgA8IAf!&ln=org.richfaces.images);

}

supply headerStyle in rich:extendedDataTable

<rich:extendedDataTable id="theTable" value="#{myTableBean}" var="stat" headerClass="my-edt-tbl-hdr" ...

but it results in following HTML

<div id="j_idt64:theTable:tfh" class="rf-edt-tbl-hdr my-edt-tbl-hdr">...</div>