Dark style preference behavior
We've discussed various aspects of this in different places, but we don't have a comprehensive plan for what this would look like system-wide and in individual apps.
Global Preference
A global switch in Settings that toggles between dark and light as the system default. This would only affect apps that opt in to the behavior, so we'd need to explain this in the settings panel in order to avoid confusion.
This switch would not affect the shell, which would continue to always be dark.
In-App Preferences
There are a number of different categories of apps, which would handle the global preference differently:
Utilities
Most system apps and utilities such as Files, Document Scanner, Document Viewer, Calculator, etc. would always follow the global preference (no separate in-app preferences).
Apps you spend a lot of time in
Text editors, terminals, domain-specific productivity apps (e.g. Builder, Inkscape, Blender) and the like often already have their own color settings, which may be more complex than just dark/light (e.g. a writing app might a sepia mode in addition to dark/light).
These apps would continue having their own settings, but add an additional option to follow the global preference (perhaps the default?).
Media Apps
Video players, image viewers, etc. would always be dark, same as today.
Apps where dark/light is part of the functionality
Apps that do things like preview icons or UI widgets (e.g. Icon Library, Widget Factory), where the dark/light switch is part of the app's functionality would follow the global preference at launch, but have an in-app setting that applies during runtime but isn't remembered across restarts.