The theme of success and failure is examined through Mitty's inability to live a fulfilling external life, which causes him to retreat to an internal life full of images of conquest. Walter Mitty is neither exciting nor successful in his everyday life. In fact, the world Mitty lives in seems hellish to him
This means that the United States has always been a commonly desired place for foreigners to immigrate
To my interpretation, it means the light from the sun is shining through the clouds. :)