It's not something that's commonly said. As to it's meaning, it depends on the context. It could just mean that you're happy it's morning, or happy that spring's arriving. Someone might use it as a replacement for something like "Praise God," which basically means you're happy for something that happened. Someone with a new-age or nature-based religion might say that and mean it literally, as in they're literally worshipping the sun. But again, it's not very common at all.

I think it's a reference to a video game - "Dark Souls" (If this is a nerd test, I may have just failed it!)