Per-app style preferences are primarily useful for text editing apps, or apps which users use for long periods of time. In these cases, the style preference can be useful as a way of controlling visual contrast and brightness.
Apps that don't fit this description generally shouldn't have an in-app theme switcher, but just follow the system setting instead.
From the UI styling guidelines:
Apps that don't fit this description generally shouldn't have an in-app theme switcher, but just follow the system setting instead.