Hello to everyone.
Please, can someone advise me on how to add this background in BS?
I`m not a developer and I’m just starting to learn.
Thank you
Well hmm where to start addressing this. They give you the code you need and they show you what component you need to use (could be any component like Column, Row, Etc.) and they give you the CSS to add. You just need to create the Div they show you in the one box, and add the CSS to your custom CSS file. I usually name mine custom.css so I know exactly what it is.
Not much else to tell you on it. I would really highly suggest you get some experience with HTML, CSS/CSS3 so you can really use this app correctly. It’s not a drag and drop editor as many seem to think it is. Other than that, let us know how it goes. 
I can say that I have the experience, not a pro, but can make a static site… When I work with Atom, everything works well, but when I use BS I dont get what I want. Im just confused. For example, why these CSS lines are crossed out?
It just not works when I add CSS in BS.
I Will try to find a solution. If I get it, will post it here.
Thanks @jo-r
You are pasting the code block for the css that you copied inside the class name background that is on the ul.
Select the ul with the class background and then Create css and then select background class (highlights blue) and then paste the code.