Can't lighten the scene after making a game.

I completed a beginner game tutorial I found here: The movie came out too dark. But I can’t seem to add lighting or change the world background. When I do and press f12 to render, nothing changes in the scene. I tried to do this in Blender Game mode as well as Blender Render mode. Does anyone know what I can do to lighten up my scene before I go through the game simulation and recording process again?