Apple Numbers Sum in category summary line won't allow red text for negative numeric values

In Numbers 15.3.1 on a MacBook Pro M5 using Tahoe 26.6.2 how can I set a negative number to red text in a Sum in a category summary line?

Google comes up with various, and often contradictory, methods none of which work.

The main reason is that a numeric Sum field in the summary line does not allow conditional highlighting.