I first visited this topic last year, but just gave a quick overview of a plugin you can edit. Since the process is a wee bit different in 2.7, I thought I’d just go ahead and release a quick plugin for you to play with.
Grab the zip here Custom Login (3850), unzip it in your mu-plugins folder. You will have:
- custom-login (folder)
Replace logo.jpg with your own site logo. If it’s a different name, edit custom-login.css to reflect it. Everyone on the system will see your logo when they log in. You can add more styling to the included css file if you wish. Take a look at the standard login screen’s css, paste in what you need with the changes you want.