Semi-Transparent menu over full page slider.

A while a go I had an issue with making a “Fullscreen Slider”, cover the entire page and have the menu semi-transparent.
(https://kriesi.at/support/topic/enfold-header-transparency-with-full-page-slider/?login_error#post-212091).

The fix was to add:
slide_height = Math.ceil( (viewport / 100 ) * this.options.height ) + 88; <— to the shortcodes.js
and some custom CSS code.

Since the update, there is now an option to have a transparent menu. This is great, but I’m unsure as to how to make the menu semi-transparent?

In the past I used:
.header_color .header_bg {
background: rgba(0,0,0,.5);
}

Now it seems to either be fully transparent or not transparent. How do I make the menu semi transparent and have the underlying image sit underneath it, so the menu sits on the top of the image? (the +88 trick no longer works).

Thanks