Select the range.
On the Home tab of the ribbon, select Conditional Formatting > New Rule...
Select Classic.
Select 'Format only cells that contain'.
Leave the first drop-down set to 'Cell Value'.
Select 'less than' from the second drop-down.
Enter =0 in the box next to it.
Click Format...
Activate the Number tab.
Select Custom.
Enter ;;; in the Type box.
Click OK until you're back in Excel.