You can override the background-color property in the .ngHeaderCell class declared in ng-grid.css to use the background color you want in the header cells.
If you do not want to modify the original ng-grid css, you can create your own css and load it after ng-grid css, in which you can subsequently override the same .ngHeaderCell class using:
.ngHeaderCell { background-color: [your background color] !important; bottom: 0; color: [your foreground color] !important; position: absolute; top: 0; }
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