Blogger full page background image

Handre Plain as my background image taken from Uriminzokkiri gallery

I googled to find the code to put a fixed full background image for this blog's desktop version as follows:

Fixing my background image and website layout on Blogger using CSS or HTML

I'd like [the background image] to be full size and remain still while the content scrolls.

I've tried the CSS suggestions I could find when searching this topic here but they do nothing when I add them to the "Custom CSS" on Blogger.

Is there a better way to fix this using HTML or is there something else I need to do to get the custom CSS to be recognized?

Put the following code in ADD CSS box.

.bg-photo-container, div.bg-photo-overlay {
  height: 100%;
  position: fixed;
}

Source: StackOverflow

The path to the ADD CSS box:

1) Admin Panel
2) Theme
3) Customize
4) Background -> Background image -> down arrow button -> Select background image -> Upload image -> Choose file -> Done -> Apply to Blog
5) Advanced (scroll bar in the right side -> down to bottom)
6) Add CSS (input the code as above-suggested)
7) Apply to Blog

That's it for me. Thank you, StackOverflow!