They’re similar, but one has a bit more molasses.
Your recipe forsweet potato casserolecalls for dark brown sugar, and you only have light.
Caitlin Bensel; Food Stylist: Torie Cox
What Is Brown Sugar?

Credit:Caitlin Bensel; Food Stylist: Torie Cox
The darker thebrown sugar, the more molasses it contains.
The Difference Between Light And Dark Brown Sugar
Light brown sugar usually contains 3.5 percent molasses.
Dark brown sugar has nearly double that amount, usually containing 6.5 percent molasses.

Credit:Caitlin Bensel; Food Stylist: Torie Cox
This helps dark brown sugar have a deeper, more complex flavor with prominent toffee or caramel notes.
It’s not something you should have to mitigate for in a recipe.
Can I Replace Brown Sugar With White Sugar?
This substitution is a little more problematic.
Baked goods made with brown sugar tend to be more moist, denser, and chewier.