Watching tutorial video in here: https://www.youtube.com/watch?v=QBXDSQuE-J0&list=PLZSqbv2i8LOD_QjTa-YlD39PsbUG-gJt_&index=8​

Go to Theme settings > Layout

1 - Site Width

1.1- Full Width

1.2 - Boxed

1.3 - Content Full Width

1.4 - Wide 1600px

2 - Site Background

Note: Only set the background color & image with "BOXED" layout