One of the themes of the novel concerns the idea of looking past the surface and past first impressions.
Boo Radley is the primary example of a person who is misunderstood as a result of a popular set of rumors.
Boo Radley is labelled as a monster and outcast because he is not seen outside his home.
As Scout and Jem mature, they realize that the rumors are not true and that Boo Radley is anything but the rabid, feral man the rumors make him out to be. They learn to see past the surface and see what is really true.
Radley is not the only example of this theme. Mrs. Dubose and Dolphus Raymond are also figures that present a false appearance.
Raymond offers a symbol for this theme in his encounter with Scout and Dill. He is believed to be a drunkard who sips whiskey from a bottle hidden in a paper bag. The children are surprised to discover that the bottle does not contain whiskey, but Coca-Cola.
He fosters a reputation as a drunk to give townspeople a reason to excuse his flaunting of social taboos.
The theme of looking past surfaces is clearly demonstrated in this scene, as is a further theme relating to social/normative pressures in small town life.
As far as themes...
1) People fear the unknown. The people of Maybcomb are genuinely scared of Boo and of change, really. We can see from their attitude toward the trial that they are scared of the allegations Atticus was implying: A white woman had interest in a black man. Not the other way around.
2) Innocent things can be corrupted by outside, evil forces. This one is pretty self explanatory.
And yes. Symbols in this novel DO reflect the themes. For example, the mockingbird refects theme #2. Boo reflects theme #1 (Tom does a LITTLE bit, but not .. just use BOO :) )