Reply To: Create a flexible content with various widths in the back-end

• Thomas

• October 30, 2018 at 9:12 pm

Use the admin_enqueue_scripts action like this

function load_custom_wp_admin_style() {
        wp_register_style( 'custom_wp_admin_css', get_template_directory_uri() . '/admin-style.css', false, '1.0.0' );
        wp_enqueue_style( 'custom_wp_admin_css' );
}
add_action( 'admin_enqueue_scripts', 'load_custom_wp_admin_style' );

More information you will find in WordPress codex https://codex.wordpress.org/Plugin_API/Action_Reference/admin_enqueue_scripts