How do you change the background of a premade layout?

This post talks about how to code in a custom background for your profile. I suggest looking for those specific code lines in your layout code and from there, you can just follow the post's instructions.

Usually, layouts would add comments in the code to organize it but if the layout you have doesn't have comments, you could also just check a layout that does and search for its background code to find what the code looks like

TYSMM! This is wonderful advice and that post was exactly what I needed! ^^