How is "Fair and foul and foul is fair" theme of the play macbeth?

This is a statement of equivocation or ambiguous expressions, especially in order to mislead. The witches loved to do that stuff. They dealt in riddles to mess with the weak of mind like Macbeth. You are correct that we can follow this particular chant to a theme. There are two sides to Macbeth. He is both good and bad, both "foul and fair." The witches symbolically throw Macbeth's nature up in the air to see which path he follows. Macbeth chooses the foul path.The foul path is littered with fair people who have gone astray. Macbeth has goodness in him but once he chooses the path of darkness, he becomes what he once feared.