Is there a practical difference between light and dark brown sugar? I ask because I've seen many recipes specify one or the other. Do they really behave differently in some way?
They have a different ratio of white sugar to molasses.
Therefore, dark brown sugar is more hygroscopic, and will have a deeper molasses flavor (and color, obviously) They're pretty similar, and you can usually get away with replacing one with the other, but if you want subtle flavors coming through, dark brown sugar might mask it.
I've seen recommendations that call for substituting light brown sugar with a 50/50 mix of white sugar and dark brown; I don't know how messy (or if even possible) to add molasses to light brwn to get dark brown sugar.
This article tells you all you need to know about brown and white sugar and cookies:
Cookie Fact #9: White Sugar = Thin and Crisp, Brown Sugar = Tall and Moist A mixture of the two provides a good balance, and as I noticed in my egg tests, dissolving too much sugar can lead to a texture that's too uniform. With sugar left in distinct grains, the pockets of melted sugar that caramelize within the cookie as it bakes remain irregular, giving the cookie more textural interest.