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