How to add custom CSS for WordPress admin panel

If you need to place a few CSS lines for the admin panel, then edit the file functions.php at the root of the theme, and add it to the end of the file:

add_action('admin_head', 'admin_styles');

function admin_styles() {
    echo '<style>
        .update-nag {display: none !important;}
    </style>';
}

Instead of the .update-nag {display: none !important;} paste your actual values.

If you need to include a CSS file, in the same place in functions.php add this one:

add_action('admin_head', 'admin_styles');

function admin_styles() {
    echo '<link rel="stylesheet" href="newstyle.css" type="text/css" media="all" />';
}

In this case, the newstyle.css file must be placed in the same folder, next to functions.php. You can also set the path from the site root to another folder where you store CSS: /wp-content/themes/yourtheme/folder/