...
Create an image that shows your logo on a coloured background, with the proportions 3:1. Place the logo in the middle, leaving coloured space above and below it.
Go to the Theme Editor > Main Appearance and make the following settings:
Background: upload your image
Image options: full
Position image: centered
Banner height: the same height as your background image, or half of it.
In the Theme Editor > Header, set the background colour to transparent.
In the Theme Editor > Start page, toggle on Fill background.
Save the theme and go to the home page of the site you applied the theme to.
Edit the layout, and:
Make sure the section is set to Transparent.
Add a Search Highlight or Navigation module
Set the module’s height to match the height of your background image. In our example, this is 500 pixels.
...