proper way to add css using functions.php?

Your add_action() is incorrect:

add_action('wp_enqueue_scripts', 'wp_enqueue_style');

The 2nd argument is supposed to be the function that you enqueue your stylesheet in. In you case that’s fearnothing_script_enqueue():

add_action( 'wp_enqueue_scripts', 'fearnothing_script_enqueue' );