How to override wp-admin styling

As noted by @Fusion, this has changed beginning with version 5. In order to override admin styles for 5.*, you need a hook in functions.php, something like this:

function custom_admin() {
    $url = get_settings('siteurl');
    $url = $url . '/wp-content/themes/my-theme/wp-admin.css';
    echo '<link rel="stylesheet" type="text/css" href="' . $url . '" />';
}
add_action('admin_head', 'custom_admin')

Sources:
WordPress Codex
isitwp Snippet

Please note the the isitwp Snippet erroneously mixes html entitites with angle brackets. You should use angle brackets not entitieis.