I believe this is not possible.
The reason is that when a service check times out, you have no idea what the
result would have been if you had got a response, therefore the status is
Status “critical” means “we know what the result is and it’s not good”.
I can only suggest that you ensure your alerting and notification systems treat
status=unknown in the same way as status=critical.
If you want status=unknown to be the same red colour as status=critical on the
service check grid, you can adjust these colour to your heart’s content by
creating your own theme for Icingaweb2:
mkdir -p public/css/themes
tells you more about how to define the colours.