Please actually read that information.
You do not edit any file, per se. You would need to create a new file. Either a plugin, or add information to your themes functions.php. I do mine from a plugin, so that it is not theme dependent.
You must completely write the new admin theme from scratch. Or use a premade plugin
Often, a tool such as firebug, or the dev tool built into chrom can help you find out the name of each part of the admin interface so that you can write your new css sheet
If you were to simply go in and edit the core WP admin styles, those changes would be lost at some point on a future WP upgrade.