Help with making Background a photo - Dawn theme

[Hd24]

Hello guys, I hope all is good. I need help regarding my shopify website. I been trying to make my websites background to be a photo, but keeping the header/footer/cart and menu page to be the theme color, however no matter what I do the background still stays the same and the changes won’t apply. I tried almost every code possible, spoke with support, but no matter what code i apply it doesn’t work, the file of the photo is below. And my website name is veiledhayati.com, please can anyone help?🥲

https://cdn.shopify.com/s/files/1/0828/0883/2291/files/floral_6a6dfd34-541b-4f3e-b7ac-b1aeeca6a641.j..."

[Hd24]

Hello, wheres the code? Also, i do not have a theme file in the assets folder, i only have base.css in there. I have a theme.liquid file in another folder i believe the layout, but not assets, so which do i paste it in? I have used codes like this before but no matter what the background still stays the theme color