NULL formatting and bg in base64

Hi pappu687,
I like very much your theme, i modify only 2 things.

1) background in base64 string so external image it is not necessary (vrana made this change and i agree with him)

2) format NULL value 
![image](https://cloud.githubusercontent.com/assets/1894578/5455213/ec58b862-8539-11e4-8b36-303a2d137f12.png)