Index page background image in 7.0? (NOT page/section banners)

Site URL: https://www.apertureconsultinggroup.com/home

I'm working with a client-installed template using Brine, and I want to insert a background image on all the main pages. But the template uses Index pages and I can't find a way to set a background image for the entire Index page, as opposed to the banners for each "page"/section. Here's an example with the background set for the top section.  You can see it's cut off - Instead I want the background to fade out gradually behind the first 2-3 sections.

I know I can accomplish this by combining sections/pages so there's one banner for all the content, but that's not ideal at all. I've attached a Figma screenshot of what I want it to look like. 

Add to Design > Custom CSS

body#collection-60d204608d9a33537c7a759a {
    background-image: url(https://images.squarespace-cdn.com/content/v1/60d204568d9a33537c7a72da/1645755511799-RJT2VBVKBS2XSFEPTI4D/contour+background.png?format=1500w) !important;
}
body#collection-60d204608d9a33537c7a759a section {
    background-color: transparent !important;
}

Yes!! Thank you so much @tuanphan! So simple but now I know 😁 I added background-repeat: no-repeat and it's exactly what I needed. You're the best. 

Edited for anyone finding this later - I actually used the following, so the image would scale to the width of the window but not repeat either horizontally or vertically:

background-size: 100%;
background-repeat: no-repeat;