WordPress have done excellent job in branding their product which is very evident on the log-in page. When designers deliver products to their clients they should deliver a client branded product and not a WordPress branded product.
While WordPress have made it possible to customize the log-in page and it can be cumbersome and you may not achieve the presentation that you desire.
The customized log-in page below (from this site) was created by the ‘Themed My Login’ Plugin which allows you, in seconds, to create a WordPress Login that pulls from the style sheet of the site’s theme.
I highly recommend this plugin…my clients love it!