src/vs/editor/standalone/browser/standaloneThemeService.ts/isBuiltinTheme
function isBuiltinTheme(themeName: string): themeName is BuiltinTheme {
	return (
		themeName === VS_LIGHT_THEME_NAME
		|| themeName === VS_DARK_THEME_NAME
		|| themeName === HC_BLACK_THEME_NAME
		|| themeName === HC_LIGHT_THEME_NAME
	);
}