Lohri theme parties usually demand traditional dressing but you could authenticate it by specifying traditional Punjabi attire that usually includes patiala salwars and short kurtas paired with phulkari dupattas and parandis for women and a Punjabi salwar and kurta with colourful juttis for men. Keep a prize for the best dressed to ensure everyone takes the theme seriously. You can also keep prizes for best jutti, best parandi, etc.