The theme of Macbeth is the relationship between gender and power.
You might also want to consider the roles of the witches in relation to gender and power. They are often referred to as "the weird sisters," but are just as often (when the play is staged) considered gender-neutral as participants in the play, since their "male" or "female"-ness isn't the point of their existence in this story, but rather it is their supernatural/evil participation in the plot.
Consider how the play might be different if you have a cast of men playing the witches. Would their actions seem more threatening, more powerfully persuasive to Macbeth? Or, if the cast is all women, is there more power to be utilized in their female gender? Do they, for example, woo Macbeth with their sexual wiles?
By the way, the idea of power simply being an overt, warlike, masculine expression of dominance is certainly not the only type of power to consider. Consider the amount of power that Lady Macbeth holds over Macbeth and whether she might be using her relationship with him as his wife, her sexual wiles, to influence him.
Power in drama is often found in the conflict between individual characters in the play. So, you should read the play carefully to note where gender seems to influence the way in which a character gains or loses power in a scene. For example, what role does gender play in Act IV, scene ii, the murder of Lady Macduff?
I suggest, if your essay is to be only 150 words, that you track one character (or group of charcters if you choose the witches) through the play and note how gender affects their interactions with others in moments of gaining or losing power. Good luck!
You don't ask a question in your question, you just give a topic, so I don't really know if you're supposed to agree or disagree, explain, or use the prompt as a thesis statement. Also, the 150 words are up to you. It's your assignment. You have to take the information given and write your own assignment answer.
Gender and power certainly do not constitute the theme of Shakespeare's Macbeth. They constitute one theme--one idea or issue raised--in the play.
Males possess the power. Lady Macbeth would like to. She would alter her gender if she could. She wants to be an aggressive, powerful warrior and ruler, but she is limited by her gender to using her husband to achieve power.
Lady Macbeth is supposed to be a good wife, be a good hostess, and be a good mother. She relishes the role of wife (possibly only because her husband is her path to power, we don't know), but plays hostess only to set a trap for Duncan, and rejects the love a mother should have for a child in favor of her ambition for power.
Lady Macbeth is intelligent, a planner and organizer. Born a female, she longs to reverse roles and be a male. When she comes as close as she can by manipulating her husband into murdering Duncan--her only means of power--it backfires. Macbeth shuts her out of his decision-making process and causes his own downfall, as well as hers.
She, figuratively, is a man trapped in a woman's body. She, literally, is a woman trapped in a man's world.