Not all apps support dark mode, but OnePlus could allow its users to force the feature on more apps. XDA Developers discovered code within the Settings app of the latest OxygenOS Open Betta 11 version for the OnePlus 7 and OnePlus 7 Pro that hints at forced dark mode coming to a future update. It is unclear though what apps might support the dark mode or when it might arrive.
The idea behind this option is said to be similar to DarQ, which lets you enable Android 10's built-in forced dark mode on a per-app basis. So, you can decide to retain the light theme for apps that don't look well to enforced dark themes, while keeping it for others,