I've had to apply a custom theme on several implementations and end up referring to the community each time for the how-to steps because it's done so infrequently. Below are the steps that have worked for me, hope they help.
Thanks for sharing Eric! This is a good method when users shouldn't be allowed to pick another theme, and when you want to enforce a single theme across all users. I've added a link to this article to a slightly different method of assigning custom themes to a subset of users based on company, found here Defining a system theme for multiple users
