How do I make my background gradient repeat?

Hi

On the webpage I am doing http://www.maverickartspublishing.com I am trying to work out how to make the gradient go to infinity on the left and right. I have seen this done before on other websites using a tall thin image of the gradient that is repeated but can’t work out how to do it myself.

My theme is not one of the default themes, so not 100% it will be done the same way as you normally would…though I have no idea!

Any help would be greatly appreciated.

Thank you!