How to add CSS code to admin dashboard via Child Theme e.g. modify this H1

Please add this function below to your  How to use Child Theme? How to change files (php / css / js) and keep theme future update compatibility? functions.php file. You can insert there also your own custom CSS code (example code will change h1 (main heading) font size to 100px): 

function admin_style() {echo '

<style>
h1 {
font-size:100px!important;
}
</style>

';
}
add_action( 'admin_head', 'admin_style' );