Transparant page with twenty eleven

I want to have parts of my website transparent, just like https://hzhenk.blogspot.nl/

There are a lot of posts about changing colors, using opacity etc, but still I can’t find the proper settings.

My website : https://www.henkmassink.nl/

style.css :

#main {
clear: both;
overflow: hidden;
opacity:0.1;
padding: 40px 0 0 0;
}

Does anybody has some tips?

Henk