You could have a global setting which is used by all apps supporting that theme.

I would like to see developers strongly encouraged to support a light and dark theme via system-wide setting. My preference would be for default colors to exist for each for when developers make no active effort to do so. If your app looks bad with the defaults, that's on you.

A kinder, gentler approach would be for the global setting to only apply if the app supports the user's preference, but big commercial apps often don't support themes now, and probably wouldn't then.