Your thesis statement is going to depend on what claim you want to make about the theme of racism in Othello. What stance are you taking? Are you going to claim that the play is racist? Are you going to look at the idea that race affects the relationships of the main characters? Are you going to claim that things would have been different for Othello had he not been black? All of these would be defensible ideas. However, before you can formulate a thesis statement, you have to know what you want to say about the play and why you want to say it. Remember, a thesis is a defensible opinion. What's the main point you want to get across? Your thesis should emerge from there.