I have the css I have used in the past with the classic builder to change the color of the background and font of group headers in table views but cannot seem to get it to work in Nextgen. This is the code I used in classic builder:
Give this a try: (replace view_180 with your view)
/* Level 1 group header: background and text color */
#view_180 td[data-testid="grouping-cell-0"] {
background-color: #848484 !important;
color: white !important;
}
/* Make the group name link and the arrow white too */
#view_180 td[data-testid="grouping-cell-0"] a,
#view_180 td[data-testid="grouping-cell-0"] button,
#view_180 td[data-testid="grouping-cell-0"] span {
color: white !important;
}
/* Keep the record count badge readable */
#view_180 td[data-testid="grouping-cell-0"] [data-testid$="nested-rows"] {
background-color: rgba(255, 255, 255, 0.25) !important;
}