Can I substitute light or dark brown sugar instead of white sugar in making fruitcake?
When substituting brown sugar for white sugar, be mindful if you are measuring by volume. Brown sugar packs down a lot and you may end up using more (by mass) sugar than you intended.
Your cake will be tasty. You may notice a slight texture difference, but it should be fine.
Alas, it will still be a fruitcake.
It'll be fine as long as you're cool with the molasses flavor.
In addition to the slightly "darker" flavor, you should expect the result to be more moist than with ordinary sugar. You may find you'll need to reduce the moisture in the recipe, or cook it longer.