Background theme in Blogger can be changed by going to "edit html" settings. 1) you have to find a good theme online btemplates.com is a good site. 2) download the zip file into your PC 3)upload the .xml file to blog "edit html" and save it optionally you can preview it before saving Hope it helps you
you can change layout anytime you want. just background that are simple and clean-looking because it only requires minimal editing.
I would suggest you to visit bloggerstyles.com . It has lots of blog templates for free and it also includes tutorials on how to use those free templates.