https://elgg.org/plugins/3265187 has this feature

Thanks a lot for your help! Much appreciated. :)

It worked like a charm, and now I have an understanding of how to alter views. Now I just have to always find the right pages to alter. ;)

Bonus question: Would it be possible to add CSS code into such a view? Example:

echo elgg_view('entity/edit/header', [
    'entity' => $blog,
    'entity_type' => 'object',
    'entity_subtype' => 'blog',
    CREATE HERE A CSS INFO, E.G. MAKE THE BACKGROUND OF THE AREA BLACK
]);

Sure.

Just edit this code on:

echo elgg_view('entity/edit/header', [
    'entity' => $blog,
    'entity_type' => 'object',
    'entity_subtype' => 'blog',
    'class' => 'my_class',
]);

If you continue to use this plugin then there is this file

\mod\my_plugin\views\default\my_plugin\style.css

Add your custom CSS style  my_class   to this file (I hope you know CSS: you can use SCSS too).

Use Developer tools aka Inspector in your browser to make your CSS style better.

Run 'Upgrade' via 'Administration' after saving changes to clean the caches.

Learn more.

I was wrong but I'll leave the previous reply to understand how to change CSS styles.

Well.

Your example will only change the header editing area, but not the blog preview with the header.

If so:

Add this class to \mod\my_plugin\views\default\my_plugin\style.css

.elgg-entity-edit-icon-wrapper {
   // make stuff
}

Use Developer tools aka Inspector in your browser to make your CSS style better.

Run 'Upgrade' via 'Administration' after saving changes to clean the caches.

Hello, please help with the recommendations to my above query

Can you explain in more detail what you want to do? 

What fields and for what entities do you want to import?

Is it from another Elgg instance?

If so, are your Elgg versions the same?

Do you want to fill the fields of existing entities or do you need to create them first?

Perhaps we can figure out what you need.

But in any case, you'll need to either create the plugin yourself or hire a developer, as there are no ready-made solutions yet.