How to highlight lines with shortages in customer order lines?

The only way you can do this calculation would be to create a custom field that contains the logic to do the calculation and return a boolean answer.  Then perform the conditional formatting on that column.  You can’t do calculations or references to other columns in the context of using conditional formatting.  Additionally, conditional formatting goes away in Aurena, so anything you come up with as a business process is not likely to continue in the future.