Heat oil in a large skillet over medium-high; brown beef on all sides in batches, about 5 minutes per batch. Transfer to slow cooker.
Sauté onions and peppers in same skillet about 5 minutes until softened; add garlic and cook 1 minute more. Transfer to cooker.
Add crushed tomatoes, kidney beans, and beer to the slow cooker.
Mix in chili powder, cumin, cocoa, oregano, cayenne, brown sugar, salt, and black pepper thoroughly.
Cover; cook on LOW 8-10 hours or HIGH 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.