Is Emily's keeping Homer Barron's corpse a metaphor for her inability to let go of her glorious past? Also, what is the story's theme?
the Griersons were once rich but unfortunately after the civil war, they have become poor and i'm wondering if emily keeping homer's corpse somehow related to the "fall of the Griersons"
That is an interesting interpretation of the story since Miss Emily is clearly clinging to the earlier "glory" of her family. But I do not think this is a sustainable interpretation. I would agree that keeping Homer in her bed is an attempt to hang onto something, though, and that something is love. All the circumstances of Miss Emily's life conspired to prevent her from having love. Her father found all potential suitors unsuitable, and even after he died and Miss Emily begins to see Homer, people in the town who think this a disgrace intervene to the point of consulting the minister, who gets in touch with Miss Emily's cousins. The cousins, they believe, should be able to put a stop to this unsuitable relationship. The only way Miss Emily is able to convince herself she is loved is to kill Homer and put him in her bed.
If Miss Emily were trying to hang onto or restore the earlier, more successful days of her family, would she have chosen a Yankee construction foreman? Such a man would have been frowned upon by her father and all those in the same class as her family. Did she deliberately choose a man whom her father would have considered unsuitable? Was this her revenge against her father, who prevented her from marrying? Perhaps. But certainly, she chose a man who showed an interest in her and whom few people would miss if he disappeared.
The theme of the story could be one of several, I think. One theme is the fall of the plantation class after the Civil War and the damage that plantation class did to its progeny, who were unable to escape or change when life changed in the South after the war. Women particularly lacked the wherewithal to adjust to the changed circumstances, having been cossetted, pampered, and only groomed to marry and manage plantation households. Gone with the Wind notwithstanding, Southern women were singularly ill-suited to carry on in a new world. There are other supportable themes in the story, and perhaps you will want to consider an idea of your own.