Override load-styles.php with admin screen css

If its just that one style you want to edit, you could try…

add_action('admin_head', 'admin_styles');

function admin_styles() {
    echo '<style>
        #poststuff #post-body.columns-2 {
            margin-right: 300px;
            dispay:flex important;
            flex-direction:column !important;
        }
    </style>';
}

Or add a separate stylesheet with something like this…

add_action('admin_head', 'admin_styles');

function admin_styles() {
    echo '<link rel="stylesheet" href="https://wordpress.stackexchange.com/questions/314539/style.css" type="text/css" media="all" />';
}