Light brown sugar has less molasses and a more delicate flavor than dark brown sugar; either will work in this recipe. To resoften hardened brown sugar, seal it with an apple wedge in an airtight food-storage plastic bag for a day or two.
These bars will be a lot easier to cut and the edges will come out cleaner if you line the baking pan with foil before pressing in the crust. Use enough foil so that it extends over the edges. After baking and cooling the bars, use the foil to lift the bars out of the pan to cut.