Adding background image to section with custom CSS using DeGraw template

Site URL: https://holisticbirthandyoga.squarespace.com/

Hi there, 

I'm an amateur coder so I can't say for sure - but it seems like Squarespace's updated templates don't have collection ID's like they used to? I'm trying to change out the backgrounds on certain sections throughout the site i'm building for a client. The Custom CSS code i'm using isn't changing anything - can someone help? The template I'm using is DeGraw. The site URL is https://holisticbirthandyoga.squarespace.com/ and the password is 'mindbodyseva'.

And here is my current code: 

div#yui_3_17_2_1_1582161363093_67 {
    background-image: url("https://static1.squarespace.com/static/5e47144cdeb0101ed01307c5/t/5e4db9345e0c794068e4a3ab/1582152001491/Background_V1.jpg")
}

As a follow up question - Above I'm trying to switch the background to a whole new .jpg, but if someone knows how to code in transparent .pngs that lay over the default Squarespace background color, the code for that too would be very helpful. I feel like the background may have a chance of being more high res that way. 

Thanks so much for any help you can offer! 

It still uses collection id on body tag

don't use #yui, It will change when you reload the page

Each section has different data-section-id. You can target this. [data-section-id="abcxyz123456789"] {css here}