hey christian,
you are the best!!! fast and it works perfectly 
addendum: i just now see that the content area with your code is also transparent in the published site. in the preview i in chrome it looked semi transparent with this code:
.x-container.main:before {
background-color: rgba(255, 255, 255, 0.4) !important;
}
but that does not work.
now i tried this:
.x-container.max.width.main {
background-color: rgba(255, 255, 255, 0.4) !important;
}
but that does not cover the whole width of the navigation bar.
any magic snippet to make it perfect?
thanks also for the video! i was trying to find my way around the same way. and i see in your video that the code of the this area (.x-boxed-layout-active .site, .x-boxed-layout-active .x-site) is not being highlighted when you select it. so you might know what to look for, but i just could not find that bit of code
it was much easier in “icon” by the way where i did the same.
anyhow
thanks again and have a great sunday! kai