You can change thesis theme background color and size in Wordpress blog to do this first you need to go to your word press admin. And then you have to click custom file editor. There you will see the "Thesis custom file editor". Now you have to select custom.css in edit selected file menu. Now you will see there a code box. Here you will have to past the background changing code. The code is showing in the video you can type it and then copy and paste it in the place of the code box which is showing in the video. Now you have to click the save button and you're done.