Getting "-apple-system" font for Classic Editor

Yes, I did use this one and I replaced it again once more to be sure but still no change.
Please check the testing site directly, thanks.

Can you try the updated code in that article?

Let me know :)

Sorry, don't well understand, what article?

I did try already, and currently running, the code you provided here and, as I said in my previous message, it still doesn't work.

To be clear, this is the code currently in use:

add_filter( 'generate_editor_styles', function( $editor_styles ) {
    $editor_styles = array( 'assets/css/admin/editor-style.css' );

    return $editor_styles;
} );

Ah, sorry - this article: https://docs.generatepress.com/article/classic-editor-styling/

OK, I see now and can confirm that this one actually works just fine!

Many thanks for this and happy Sunday evening.

Glad I could help! :)