It would be nice to have login and register pages in same style as other site. There is at least two plugins that trys to add such functionality with different approach. Both have problems.
(1) There are just one 'shutdown' hook in the rendering process of login page, but it's called after all the html is done. So you could add your CSS link to result, but it will be at the very end of file, right after </html>. Not the right way.
You could see how such method is used in 'WP-Custom Logo' plugin ( http://www.wiso.cz/2006/09/21/new-wordpress-plugin-wp-custom-logo-10/ ).
(2) There is also 'Themed Login and Register' plugin ( http://www.jameskelly.org/wordpress-plugins/custom-login-and-registration-forms-plugin/ ), but it is also wrong approach: it replaces wp-login and wp-admin code completely. So it some terms it 'doubles' securiy risk and duplicates existing code. (This plugin has some additional functionality also, but i don't speak about it here.)
Could it more elegant solution to be requested?
In my instance of WP i just added custom stylesheet link in 'wp-login.php' and 'wp-register.php' pages. Declarations in that stylesheet file overrides standart 'wp-admin.css'. You coul see it in http://OpFor.spb.ru/wp-login.php (it is localised in Russian, but it doesn't matter here).
Patching WP core files is shurely not the best idea. It would be mutch better if there will be action hook in WP code.
Dear developers! Could you add special hook to html-header rendering code of wp-login.php and wh-register.php? It would be the best solution for login|register pages customization problems. The action hook must be in header code, right after all css (wp-admin.css link and the buil-in style addition).
If it helps, i could write a (small) patch and corresponding plugin example.