While it may seem strange that a word about something pleasurable like "luscious" comes from a word about light, the connection is through the idea of luxury and abundance. "Lux" was used to describe things that were bright and beautiful, and then it came to refer to things that were valuable and rare, like luxurious objects.
Over time, "lux" evolved into "luscious", which now describes something that is pleasing to the senses, especially in terms of taste, smell, or appearance. So, while "light" and "delicious" seem like very different things, the connection is through the concept of something desirable and pleasurable.