How to change the background color of the Hub Navigation bar in SharePoint Online (to a specific color, not dependent on the theme)?

@frob ,

In order to change the hub navigation bar color, we can go to site settings of hub site>Change the look>under the header section>Background> select a theme color to change the background color of your site header.

Note that the background of the site header(include the hub navigation bar)will depend on your site theme:

----------------------------------------------------Update Answer--------------------------------------------------

Sorry I missed your attached request, if you don't want to depend on the theme, we can add the modern script web part, insert css code to change the backgroud-color of the hub navigation bar:

<style>  
.root-42  
{  
    background-color: #B2E23B !important;  
}  
</style>  

Then republish the page to check the result:

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