Hello @Gasap and welcome! I have renamed your topic to “How to fill the first row of a table with a color?”
If I understand well your topic, the following should work
#show table.cell.where(y: 0): strong
#set table(fill: (_, y) => if y == 0 { gray })
#table(
columns: 3,
[123],[1123],[1176],
[657],[5643],[4567],
[4567],[8970],[234]
)
Table formatting is not the most intuitive at the moment, but the tutorial is pretty thorough.