One quote from Macbeth that would definitely give you substance to work with would be "Fair is foul, and foul is fair" (Macbeth 1.1). This quote embodies the concept that even in light there is darkness, and vice versa. It is unsettling to some to think that there is darkness even in the light, so this could be an interesting point to debate/discuss in your essay. Do you believe this is true, or do you think this is an overstatement? Can you find support from experts to support your claims? Use these two questions to get you started.
Also, it is would interesting to take this quote and turn it around the other way - do you find it hopeful that all darkness has some semblance of light? This could provide for an interesting discussion as well. Good luck to you. Hope this helps.