I spend much and sometimes all of my day in AnkiDroid, so it’s necessary for the sake of ergonomics and health that the UX be as pleasant, motivating, and comfortable as possible. Choosing a background image definitely goes a long way towards this, but the restriction to robin’s-egg blue for the main UX seriously detracts. I realize the specifics are a matter of opinion, so several options should be available, but I personally think a sakura pink would be most effective.
I’m in favor of selecting the accent color of the app and Material You colors.
But that needs a lot of reorganization of the app’s UI and themes. And tbf, I find it an annoying process because everyone wants to give an opinion about colors, so more time is spent on bikeshedding than on actual work.