What is the theme of the poem?

One possible theme of "I felt a Funeral, in my Brain" by Emily Dickinson is how fragile sanity really is.
The speaker of the poem explains that their brain is slowly breaking down. It's a funeral for her sense of self and her sense of reason. Where once she could trust her mind, now it's like something new—something worse than before.
The poem doesn't trace the things that led to this madness because it's clear that insanity could happen to anyone; it isn't specific to one person or one set of circumstances. The slow beat of the mourners filing in and the box being lowered into the earth are the death knell for the speaker's sanity.
Once the funeral is complete, it marks the death of the self that insanity brings.