Pages transparent?

Example: https://www.benelux-iwa.com/en

How I can do?

That’s done by setting opacity in the CSS code for the element on the page — so the site you linked to as an example has this:

.hentry {
    background-color: #FFFFFF;
    opacity: 0.8;
}

You can use Firebug to find the element selector to use in the CSS code for the opacity.