Not sure about the second question, so I will have come back to you later, but for the first question, that’s strange. Are you sure you set the background color to dark? I just tried to do that, and it worked fine with me. Try to recreate the hero bloc and set the color to a new dark color in color swatch manager, it should be fine.
I can set the color of the sticky nav with the background swatches - but I can’t find a way to set the opacity. When the color swatches are used, there is no opacity setting. This can be achieved in the global area with classes, but then the nav bar doesn’t first appear superimposed on the hero image and the visual effect isn’t the same. In Eldar’s themes, he uses solid colors for the sticky nav bars. Is it possible to make the nav bar translucent?
If the logo is a png or svg on a transparent background, and you’ve set a dark background to the hero bloc, there should be no white background to the logo.