Not sure how casual it is but I think relaxedRelaxed is another good word: "Don't worry, she's pretty relaxed about food." Merriam-Webster defines it as "easy of manner" which seems to fit your request.
"Relaxed" also comes up in a search for a translation of "fünfe gerade sein lassen" (literally "let five be even", i.e. don't insist on formalities, don't be fussy, be flexible — the German answer to your question).