Hot take: the current mobile flatness we see all around mostly serves dark patterns schemes and it's anti-user by default. It's easier to hide options within an interface that has no clearly distinction between link and the button, the interactive object and just the decoration. That helps trick users to pick something they didn't want or make them keep the defaults that works against them.
And by the way, may I suggest you try to search on the Internet what GNOME desktop environment developers did to theming with their libadwaita and flat Adwaita design. For a start: https://www.osnews.com/story/133955/gnome-to-prevent-theming...