Adding background image to Jekyll minima theme

Question

I am trying to include a background image to my Jekyll minima website that fills the whole window, similarly to the style here: https://orianna-zzo.github.io/AllinOne-html/

I have tried to implement the both the first half and the second half of the top answer here: How to render background image in a page generated by Jekyll?, but the image doesn't insert for me even after significant time looking at Jekyll help pages trying to troubleshoot. I have tried many configurations of the image url and also tried to use local copies, eg.

.hero {
  background: url('about/pic.jpg');
  background: url('pic.jpg');
  background: url('./about/pic.jpg');
  background: url('./pic.jpg');
  background: url(about/pic.jpg);    etc.
}

where 'about' here is a directory within the main repo which contains the image file. Any help would be very much appreciated!