Custom CSS Only Working When Custom CSS Tab Is Open

Site URL: https://seal-bagpipe-jk7x.squarespace.com/

I am trying to turn the content block in my collage images transparent. 

I've managed to achieve this when the Custom CSS tab is open. But when I view my site with another tab open it goes away. Then if I open the Custom CSS tab it works again. 

Here's the code I'm using: 

Quote

.sqs-block-image .design-layout-collage .image-card {
background-color: RGB(222,222,222,.8) !important;
}

Here's what it looks like when the CSS tab is open:

Here's what it looks like when the CSS tab is closed:

Any suggestions?

Website - https://seal-bagpipe-jk7x.squarespace.com/

Password - Schr0dinger1

rgb is invalid

rgb + opacity = you need to use rgba

It should be

background: rgba(222,222,222,.8) !important;