As far as i know css/admin-theme.css is auto generated from sass files, all modifications to that could be overwritten as soon as the sass files gets compiled again.
I'm trying to do exactly the same without success.
I don't know where to begin with Sass files, nor do i know exactly how to compile them.
Then I've found this file :
..admin/themes/default/sass/controllers/_login.sass
that have all the required code to properly change the background color but, again, i can't tell how this works.
If somebody can clarify this it would be really appreciated!