[Plugin: Dashboard Site Notes] Add styling to the content

The style you would need to change is “.updated.dsn .note .title”

The “correct” way of adding admin styles would be adding this to your functions.php (untested but should put you on the right track):

wp_enqueue_style( 'my_admin_css', get_template_directory_uri() . '/css/admin.css' );

and then create css/admin.css in your theme.

You could add it to the css in the plugin itself, but then it’ll just get removed whenever it gets udpated.