Margin -Top??

Hi!

I really need help about this question and sorry if it looks too silly, but i tell you that i first made sure to read the tutorials section and i can't find any too specific to solve my problem.

here it's my question:

I don't wanna change at all the current theme of my journal (component bluetiful) i only wanna add a background at the top, but course with the "normal customization center" it's impossible add the margin-top, so i went to the "advanced cc" and made a new theme, now what's the next that should i make???, i wanna keep everything as it currently looks but with the difference of the bckground at the top, i know with the function print_stylesheet() { i can do this but i'm not sure where i have to paste it, i first pasted it in the "User" text box and it didn't reported any error but when i went to my journal it was all messed up, and all the structure of the layout was gone.

So you can help me please??

Thanks in advanced