loading a custom stylesheet in a function

To include custom styles and scripts, you should be using wp_enqueue_style() like so:

add_action('admin_enqueue_scripts', my_enqueue_admin_styles);
function my_enqueue_admin_styles() {
    wp_enqueue_style('my-admin-style', get_bloginfo('template_url') . '/css/style.css', array(), 'screen');
}

You want to use this method, as it will add your CSS after the default and thus it will allow you to overwrite the WP styles.

Then, look at the source of any admin page, and search for your CSS. Make sure the URL there is accurate. If it is, the style will load.