Skies is pretty subtle about ingame religion, but if you look it's clear that most of the Arcadian cultures worship the Moons. So what does Galcian do? He bends the Moons to his will with an ancient superweapon. He subjugates what the people see as gods.
All of the Gigas have a number motif associated with their color's number. Recumen has four heads, Grendel has one eye, Bluheim has three tails, Yeligar has four arms and a head to make five, and Zelos winds up looking like a hexagon, then gains six claws. But what about Plergoth? Sure, the setting for the Moon Crystal had two dots, but that seems a bit of a cop-out. But it also has two names, Plergoth and Rhaknam!
Silver magic represents life and death, which could symbolically be interpreted as beginnings and ends. The silver crystal is the last one you acquire, but since Fina has one inside her body, it's also technically the first one you get, too. The silver crystal marks the "life" and "death" of the quest for the moon crystals.
And speaking of beginnings and ends, the game begins in the skies above Soltis underneath the Silver moon where Dyne and Vyse seize a Valuan ship headed by Adrimal Alfonso. Shortly afterwards, the first dungeon beings at nearby Shrine Island. At the end of the game, Shrine Island is revealed to be the entrance to the Soltis continent and the final boss is fought in the Skies above Soltis. In other words, the game begins and ends under the Silver moon below the silver contintent.
One of the bonus bosses, Elcian, is unlocked after you beat Galcian. It's a black Looper. The implication? Loopers, those things you've been getting frustrated at and slaughtering for most of the game so far, are the spirits of the dead.
Observation: it sure is incredibly easy to find mechanical arms and weapons that are compatible with Drachma's preexisting mount. Conclusion: amputees are incredibly common and Drachma's is the standard model.