How do I make a Dark/Night Mode?

Hey all, i have made a Game. Now i would make a Dark and Night Mode. Any1 knows how can i make this?

I would have a Moon and Sun Icon and if i click on it the BG and 1 Sprite should be change.

Hope any1 can help me =)

Easy here

http://www24.zippyshare.com/v/2zY3Jtpu/file.html

Click on Night button to make it night , click on day button to make it day.

Cool very Simple =) and how can i change the Sprite on clicked? I mean if i click sun the bg would change sun and the icon should change to moon

Ok so you want only one button to change the weather?

Cool very Simple =) and how can i change the Sprite on clicked? I mean if i click sun the bg would change sun and the icon should change to moon

Just do onclick on the sprite and use the same code I did for the buttons.

Can u give me an Example Pls?