My colleagues want a new host group to have more visibility and want the Critical status to be different color. This is defined in nagios3/stylesheets/status.css as statusBGCRITICAL but as I can't seem to modify status.cgi I don't know where to put that logic.
Is there another/any way to have different status colors for different host groups?
In my mind this is a typical XY problem. Your actual issue is that you for whatever reason need one host group to get extra attention compared to others: you then instantly assume that the best way to achieve this is to start modifying the Nagios CSS rules and add/modify additional logic to the web interface (basically forking it, and making it possibly incompatible with future code patches/changes) to make this happen.
Step back a little, take a breath and ask yourself: why does this host group need additional attention? If this host group needs additional attention when objects go into warning or critical, what ways are there for us to indicate this to the relevant people without requiring them to stare at a screen all day long and notice that one color is different from another?
A couple of things that come to mind are solutions involving notifications. You could for example send the notifications for this host group to a special e-mail alias that is automatically flagged in everyone's inbox -- or, you could send it to a specific Slack/whatever channel that everyone has notifications enabled for -- or, you could send an SMS, or a push notification - or...
There are many solutions to this issue that are much better, and work within the parameters of what Nagios should be doing, without you going into it and making third party code changes to the product.