Edit custom.css which is located in Avenue theme folder.
Edit the following code and add it to your custom.css
Code link: http://pastie.org/2724874
- transparent can be replaced with a color such as black, red, blue or a hex code (preferred)
- no-repeat can be replaced with repeat-y to replace the background vertically, repeat-x to repeat the background horizontally and just repeat to repeat it both, horizontally and vertically.