Header image and background image scale matching when changing displays

Hello.

I’m using Sydney theme on my website. I want to make header image and background image to match completely on different displays. I’ve made an image (white background and three red tripes) and put it on header image and background image. I’ve used some custom css code (found it somewhere on this forum) to make header image act as I want, now I want the background to match the header by connecting to header image. It works for me on pc display, on tablet, but not on mobile display. On mobile display those red stripes does not “connect”. Can you help me to make the connection happen? :) my website is: www.photoambi.com

the code I used for header image is: