There is a lot of symbolism is Joyce's Ulysses. I would like to focus on his use of light and dark, though. Normally, darkness is associated with the "bad guy" and light is the "good guy." For example, Darth Vader always wears black, and Luke Skywalker (and most Jedi) wear white or light browns. The motif is repeated all over Hollywood. Joyce flips this around, though. Stephen and Bloom are dressed in black, and Boylan is dressed in light clothes and has a "flashy" personality. There's other subtle color symbolism, too, for example when the "dark horse" wins the race.
A universal message that Joyce writes about is the need for multiple perspectives. It's good to view things from another point of view. Joyce is not the only famed writer to have emphasized this particular thematic idea either. Harper Lee wrote about it in To Kill a Mockingbird. Atticus tells Scout the following:
"You never really understand a person until you consider things from his point of view--until you climb inside of his skin and walk around in it."
In other words, it's important to see things from another person's point of view before you make a judgment. Joyce stresses this idea to his readers through multiple character perspectives and the concept of parallax.
Parallax is actually an astronomy tool used to measure the distances to stars. By using the apparent in shift in the position of a distant object due to the motion of the Earth, an astronomer can calculate the distance to that object. An easy way to illustrate it is to hold your thumb up an arm's length away from your face. Close one eye while looking at your thumb, then close it and open the other eye. Your thumb appears to wiggle back and forth, but it really isn't moving at all. Or imagine your family road trips. If you look at the objects close to you, they appear to be moving quickly past you, but the mountains in the distance barely appear to move at all. A greater distance has a smaller parallax. This is how astronomers gauge distances to stars and galaxies.
By shifting between three perspectives, Joyce forces his reader to come to a conclusion, and then immediately re-evaluate that decision based on the new perspective. By seeing the parallax of characters, the reader is taught that an immediate snap judgment might be a very poor judgment.