Function default_target_platform 

pub fn default_target_platform() -> TargetPlatform

The platform whose conventions an application follows unless its theme says otherwise: the debug override if one is set, else the browser’s operating system on the web, else the platform the binary was built for.

Widgets do not read this directly: they read Theme::platform, which starts here, so that a theme can change it for a subtree or for the whole application.