doubles

I would like to put a picture behind the text on all posts. I want to remove white back behind text and put an image.

I have tried several versions, but none has worked.

I have put it in body. This is how it was:
body {
background: #fff;
line-height: 1;
}

I have made this change (image has another name because of transparency):
body {
background-image:url(‘https://osivanamazuranica.hr/public_html/wp-content/uploads/skola2013-CB-prozirno30p.png’)
/* background: #fff; */
line-height: 1;
}

This is the page
I would like this image on homepage as the image on all pages.

How can I remove a right side bar on the classroom math ote061 theme?
I have tried to make a child directory, but then I`ve copied some files from parent directory, and something is wrong… I received a message that parent theme is missing and cannot bi installed?! It is already installed, of course.