I want to focus on the theme of alcohol in "The Black Cat." What should my thesis be?I'm using the title, "The Legacy of the Fiend Intemperance."

I like your title! What you need to do now is to re-read
the short story and consider what particular line you are going to take. I think it is
pretty clear if you read the short story that the narrator's behaviour is largely the
result of his alcoholism. That in fact could be your thesis statement. Now you just need
to identify quotes that support this idea, such as this
one:

One
night, returning home, much intoxicated, from one of my haunts about town, I fancied
that the cat avoided my presence. I seized him; when, in his fright at my violence, he
inflicted a slight wound upon my hand with his teeth. The fury of a demon instantly
possessed me. I knew myself no longer. My original soul seemed, at once, to take its
flight from my body and a more than fiendish malevolence, gin-nurtured, thrilled every
fibre of my frame.

You can
hopefully see how this quote supports your thesis statement. We can see the barbaric
behaviour of the narrator as being directly linked with his alcoholism. What he does to
Pluto, in cutting out his eye, is something that he does because of being drunk. The way
he describes himself makes it clear that it is almost somebody else who is committing
this crime. Note that he says "I knew myself no longer," and "the fury of a demon...
possessed me." This quote clearly supports the thesis expressed above. Now all you need
to do is find more quotes to support your ideas. Good luck!