I would start in the css rather than inserting code into your .php files
If you look in your ‘Editor’ there should be a file ‘style.css’ at the bottom.
not far from the beginning of this file you should find
.col-full {
margin: 0 auto;
width: 939px;
}
I would change that to:
.col-full {
margin: 50px auto 0 auto;
width: 939px;
}
make sure you let your developer know of any changes ??