Chrome drawn outside the theme scope

An app's own window decorations get composed before the theme, because they wrap it:

// adapted — the desktop window body
Column(Modifier.fillMaxSize()) {
    CustomTitleBar(…)     // ← here, MaterialTheme.colorScheme is the FRAMEWORK DEFAULT
    App()                 // ← AppTheme(…) is entered in here
}

MaterialTheme.colorScheme above that second line is neither an error nor null: the object falls back to a built-in light scheme when none was provided, so the title bar reads perfectly valid colours that have nothing to do with the user's choice. The fix is not a second theme — resolve the decision once and hand the chrome finished colours: