Hi Peter
Yes I did set the theme property to nedfit, but I found out what I did wrong. Below the proper styles :
.jqx-widget-nedfit .jqx-grid-cell-pinned-nedfit, .jqx-grid-cell-pinned-nedfit {
background-color: #000000;
color: #222;
text-shadow: none;
}
.jqx-widget-nedfit .jqx-grid-cell-pinned-alt-nedfit {
background-color: #000000;
color: #222;
text-shadow: none;
I forgot the .jqx-widget-nedfit part 
Thank you for your time !
Erwin