Yes, the transparency is CSS code, an alternative way of defining color rules in CSS. With firebug I've found that you've already applied the transparency for the main content in this code:
background-color: rgba(255, 255, 255, 0.8);
What did you use to write the CSS rule, a custom CSS option or plugin? If you want to add it in the child theme, simply add it in style.css in your child theme. You can add it anywhere, and it'll work, however, it's preferable to add a section that has all the styles for the main content so it'd be easier to keep track of your styles. Also, make sure that the inline style that you have added is deleted because it takes precedence over the stylesheet.