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” />’; }