apply css inside the admin panel

1Step 1:  Create Your CSS File
2You can place the CSS file wherever you'd like; I've chosen to place the CSS 
3file within my theme.
4
5Step 2:  Add Your CSS to WordPress Admin in functions.php
6
7WordPress uses an add_action type of admin_enqueue_scripts for adding
8stylesheets anywhere within WordPress:
9
10## Update CSS within in Admin ##
11
12function admin_style() {
13  wp_enqueue_style('admin-styles', get_template_directory_uri().'/admin.css');
14}
15
16add_action('admin_enqueue_scripts', 'admin_style');
17
18
19reference : https://davidwalsh.name/add-custom-css-wordpress-admin