Heat oil in a large skillet over medium-high heat. Working in batches, brown beef cubes on all sides, about 5 minutes per batch. Transfer to slow cooker.
In the same skillet, sauté onions and bell peppers until softened, about 5 minutes. Add garlic and cook for 1 minute more. Transfer to slow cooker.
Add crushed tomatoes, kidney beans, and beer to the slow cooker.
Stir in chili powder, cumin, cocoa powder, oregano, cayenne pepper, brown sugar, salt, and black pepper. Mix everything together thoroughly.
Cover and cook on LOW for 8-10 hours or HIGH for 4-5 hours, until beef is tender and falls apart easily.
Taste and adjust seasonings. If chili is too thick, add a little water or broth. If too thin, remove lid and cook on HIGH for 30 minutes.
Serve hot topped with shredded cheese, sour cream, and chopped green onions.