How to add image behind blog header, to match rest of site?

Site URL: https://elephant-goldfish-3sfs.squarespace.com/blog/jaimin-and-chris

I want to add an image behind the nav on my blog posts. I don't want it to be white, while the rest of the site has nice image backgrounds! Can anyone help me figure this out please?

https://elephant-goldfish-3sfs.squarespace.com/blog/jaimin-and-chris 

PW: trillium

I want it to look like this image:

@tuanphan Can you help me with this please?

On 3/22/2021 at 5:03 AM, vireshwara said:

@tuanphan Can you help me with this please?

Hi. Add to Design > Custom CSS. Replace with your image url

body.collection-type-blog-side-by-side.view-item header#header {
    background-image: url(https://cdn.pixabay.com/photo/2021/03/10/10/51/cherry-blossom-6084249__340.jpg);
    background-repeat: no-repeat;
    background-position: center center;
    background-size: cover;
}