Full screen resizing background

Help me !!:crying: i’m pulling my hair with this thing.
I’m making a site with pictures backgournd that resizes when the user change the window size and the content stay centered. The code was from rhamej. Problem is that the content is centered only after the window is resized. I can’t see what’s wrong and i’m becaming crazy with this …
The file is here FILE

Ubik