The theme of a literary work is its main idea. In a complex work like To Kill a Mockingbird, there are several themes, so you have some choices as far as what you want to relate racism to.
For example, if you take the theme of growth, you might consider this sentence:
As Scout grows from innocence to maturity, she has to confront the painful reality of racism.
That would be an example of a thesis statement that includes both theme and racism.
If you want racism addressed via another theme, you could address the theme of courage, and try something like this:
In this classic novel, Atticus Finch demonstrates courage by he standing up to physical threats, social pressure, and racism.
Just about any of the novel's major themes can be related to racism.