create admin panel css code example

Example: apply css inside the admin panel

Step 1:  Create Your CSS File
You can place the CSS file wherever you'd like; I've chosen to place the CSS 
file within my theme.

Step 2:  Add Your CSS to WordPress Admin in functions.php

WordPress uses an add_action type of admin_enqueue_scripts for adding
stylesheets anywhere within WordPress:

## Update CSS within in Admin ##

function admin_style() {
  wp_enqueue_style('admin-styles', get_template_directory_uri().'/admin.css');
}

add_action('admin_enqueue_scripts', 'admin_style');


reference : https://davidwalsh.name/add-custom-css-wordpress-admin