Vast HTML

This is a full featured plugin to help you edit your login just the way you want it. Lets you use or block the original wordpress 
css (the first plugin to do this). Comes with one pre made style you can use or deactivate. 
Very easy to create your own style using the style tags givin in the plugin. 
Please visit http://vasthtml.com/js/login-xchange-plugin/ for usage instructions.

Login XChange

This plugin will let you change the css of your login and register pages.

prontotools

zkancs

Do some or all of the websites in your multisite network require a custom login logo? Well now you’re in luck!
This plugin allows you to customize the login logo per site, with the option to use the network’s default logo as well.
Once installed, access your theme’s Customize settings to update the logo.
Here is our <a href="https://github.com/prontotools/multisite-login-logos" title="Fork" rel="nofollow ugc">Git repository</a>. Feel free to contribute!

Multisite Login Logos

Easily change the logo on a network site's WP login screen using WordPress's Customize settings.

Login XChange vs Multisite Login Logos