Great, those are the exact details I needed.
You’ll need to add a bit of custom CSS to fix the opacity. It’s best to add this to a new theme component (in the edit html/css > css > common section), and then add the theme component to the main theme. This way it won’t be overridden when the main theme gets updated.
.d-header {
background: $header_background;
}
There’s some additional detail here about using themes and theme components if you need help: Beginner's guide to using Discourse Themes