Posts web pages are not placed correctly

I am using the theme Magazinum for my website https://www.dicordmagazine.com, and after that i added a code to the page template to be able to add a custom background, the posts pages are not placed in the middle of the screen as they should be. They are placed on the left side of the screen, and I don’t know how to return them back in the middle of the screen.
Also the Events posts and the ISSUU pages.

So can you please help me or do you have any idea what shall I do to fix this issue?

Thanks