The novel is long enough, and with characters dynamic enough, to see why it would have more than just one theme. However, a universal theme in the novel is Wants versus Needs.
If you think about it, Silas Marner lived his life entirely based on things he thought that he wanted. In Lantern Yard, he wanted the lifestyle of his religious sect. He wanted a girlfriend, so he got one. He wanted friends, so he got those, too. He wanted to be a part of something, so he was a part of his congregation. He wanted to demonstrate his loyalty, so he became popular there. He wanted to marry his girlfriend and continue life as a happy man. Then all came crashing down after his best friend betrayed him and framed him for robbery.
Still, Silas was too blinded to realize his friend was not a true friend. His girlfriend ended up with this man who betrayed him. After the robbery his religious sect did not even give him a chance to plead his case. Were these things those that he really what he wanted? Were they even what he needed?
Anger continually festered in Silas once in Raveloe, isolating himself ever further. Never did he give a thought to the idea that, regardless of what he had once wanted, he may actually be in need of something very different. Silas just reverted to his pattern of wanting and, this time, began to want gold, collecting and counting it. Collecting gold, living on his own, keeping to himself, and becoming an antisocial recluse became his new wants but were definitely not his needs.
Mercifully, baby Eppie appears right when the gold is stolen. The golden-haired child makes an instant impression on the isolated Silas. Immediately, he realizes that he has finally found what he needs: a center, an anchor, something to base his life on. He is more than willing to take Eppie in and raise her as his daughter and devote his life to her.
This universal theme shows that what we want and what we need do not appear hand-in-hand. What we need often becomes known only as a result of a life-changing event, an event we may desperately need.