“…add another background declaration right before the existing declaration and set its value to red.”
You see the current background declaration, right?
You are going to add another one of those before (or above) the current one except it will have a value of red. The keyword red is already defined in CSS, you don’t have to do anything special to use it. Using red for the value is the same thing as using the alpha-numeric value for red.
You can have multiple declarations for the same property in a CSS rule set.