Update 17/07
I got some tips and finally solve the background color image and the mobile adjustment.
16/07
This is not my "ideal solution", but I decided to post to record my attempt. I tried with grid, but the background color image was (is) a big issue for me. I tried so many times, that I made two code files, and to post I chose to use media queries to transform the screen adjustment.
I have another code (flexbox) and maybe when I finish, I will post too.
you can use only the div with an image inside and make the color overlay using a pseudo-element, and the same code works for both mobile and desktop
I made some modifications to the index.html and style-ok.css file and sent a pull request with them.
if you don't know about pseudo-elements check these videos:
https://www.youtube.com/watch?v=zGiirUiWslI
https://www.youtube.com/watch?v=xoRbkm8XgfQ
https://www.youtube.com/watch?v=djbtPnNmc0I