Opacity and transparency of the body

You can leave it white, but add transparency. You will use rgba for that:

https://stackoverflow.com/questions/10422949/css-background-opacity

I made the body white, 90% opaque, but both the menu and the page background stay white with 100% opacity.

Is there a way to change this all at once? Or how could I otherwise have all the content on a 90% opaque white background (over the actual background image of course)

Namaste!

You can style any element of the theme, please see how to target those elements:

https://mtomas.com/389/tiny-forge-framework-child-themes-comprehensive-guide#Use-web-dev-tools