What do Piggy's specs symbolize and what theme do they reveal?
The specs symbolize one thing at the beginning of the story and another by the end. At the outset, Piggy is the one boy who understands or sees what is really happening to them. The specs are the symbol of that "vision" or understanding. He sees that they are not going to be able to expect a rescue, that no one knows where they are, that the world they've left has been destroyed or at least severely damaged.
The specs then take on another symbol, that of the boys' power over fire. They need the specs to start a fire and eventually there are several conflicts over who gets to use Piggy's specs.
Near the end of the story, Piggy goes with Ralph to confront Jack on castle rock and Roger takes advantage of the opportunity to push a huge boulder off the cliff, killing Piggy. His glasses are also smashed and in this state serve to represent the fact that the boys have completely separated themselves from whatever vestiges of civilization that remained.
As such they now represent the theme of that loss of civilization, of the way that savagery and violence have taken the place of order and rules.