body {
   background: url('https://example.com/link/to/image.png');
   background-size: contain;
   }

Do have the image uploaded somewhere? Without an actual image, there are questions about repeats, contain vs. cover for background size.

https://css-tricks.com/almanac/properties/b/background-image/
https://www.w3schools.com/cssref/pr_background-image.asp
https://developer.mozilla.org/en-US/docs/Web/CSS/background-image

I just remembered the customizer also works on the portfolio page and i uploaded a background image. But it only covers the sides! I want it to cover the whole page!

I will try your code. But I forgot to ask which file? Does it goes into page.php ?

If the customizer puts up a background image, then you just need to make the page’s content not have a solid white background. And do not edit any PHP files.

Use this CSS:

.post-inner, .page-inner, .col-main {
    background-color: transparent;
}

To add CSS: use the “Additional CSS” option in the customizer. https://codex.www.ads-software.com/CSS#Custom_CSS_in_WordPress

Learn to use the Chrome Developer Tools to help you see and test changes to your CSS.

Steeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeve!
The css works and looks killer ??
I love you man!

I am using a child theme so i am safe editing php files; but in this case there is no need ??