Can't Find Block ID?

Site URL: https://alpaca-cat-m89c.squarespace.com/

Hello, I'm trying to get the main background picture to change sizes for the mobile version of the website, but it won't change. I've looked all over for the block Id in the code for the CSS to change it, but I can't even find the block id. I even downloaded a plugin to show the id's, but it doesn't even show with the plugin. I was wondering why that's not working, and if someone could help me change it?

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