Do you want it like this?
https://prnt.sc/nokykljhU2OL
Then use this custom CSS
.sp-page-builder .page-content #section-id-1676643080592{
opacity: 0.7;
}
.sp-page-builder .page-content #section-id-1678826910496{
opacity: 0.7;
}
Path towards custom CSS: Extensions>Templates>Styles> Click on your current default template name then choose Template Options. Then find custom CSS within the custom code.
In the meanwhile you can follow these blogs, it will help you in your CSS work
https://www.w3schools.com/css/css_image_transparency.asp
https://www.joomshaper.com/blog/which-line-of-css-i-have-to-edit-code-inspector-in-action
Thanks.