Plus, it's easy, oh, so easy, to offer simplicity (the defaults) along with a few options for those (many) who want it!
Also, Apple's known for providing things for people that they didn't know they wanted! And, I'll bet that many of the things mentioned in this thread, many people would truly enjoy if they were included!
My issue with the Big 3 is that they sort of get it (e.g., Amazon's night mode, but no color themes; etc.)
Of course, related to all this is my pet peeve about dictionary/word apps-- that they don't have a night mode! It makes no sense; it's as if they never considered that people might want to look up a word at night and NOT be blinded by a bright white screen!