Body Border

What are you trying to do? Do you have a child theme set up?

https://codex.www.ads-software.com/Child_Themes

You should not make any changes to files especially in a default theme.

Sonny, I was making child themes back when you were in diapers.

But in all seriousness, yup, child theme is set-up and ready to go.

this is a basic example of a body border

I don’t have a clear strategy for doing this in twentytwelve.