I answered my own question. For anyone looking to do the same: It is as simple as uploading the custom style.css sheet into the users media folder. You'll also have to include and custom image files.

Users or visitors?

Public skin switching (for visitors yah?) is gone, but each blog can still have it's own skin. If your users don't have a blog each then that doesn't help.

So the "user and blog custom style sheets" has to be enabled in the back office under one of the Blog settings subtabs. Then you have to upload to media/blogs/blogname/ or media/users/username/ folder a "style.css" style sheet that will override the existing style sheet for the skin for the blog they are on.

An easy example is the header image in the evopress skin. "boring blue" gets old ya know? So I found and made a couple of different images, uploaded them to the appropriate media/blogs/blogname/ folders, and also uploaded a style.css file that says "background-image: this one instead of the stock one;". Blammo - instantaneous customization. Since I'm the only user there is no need for me to upload a user-specific style. If my visitors want different images for the blogs they'll have to wish really hard then suffer the emotional catastrophe of not getting what they wished for.