The background for the content/sidebar area is made up of three images, top, bottom and middle The middle repeats in the "Y" direction (down) and aligns with the top and bottom image. Here are the three image URLs.
http://s1.wp.com/wp-content/themes/pub/choco/colors/default/images/main-top.jpg
http://s1.wp.com/wp-content/themes/pub/choco/colors/default/images/main-bg.jpg
http://s1.wp.com/wp-content/themes/pub/choco/colors/default/images/main-bot.jpg
And here is where they are located in the CSS (relative links rather than absolute).
#main-top {
background-image: url("images/main-top.jpg");
}
#main {
background-image: url("images/main-bg.jpg");
}
#main-bot {
background-image: url("images/main-bot.jpg");
}
You will have to recreate those images in the colors you want, making sure they align with each other right down to the pixel and then upload them to your media library, get the URLs of those images, put the above into your CSS and then replace the relative URLs in them with the appropriate absolute (full) URL.