Your comments

Why am I not surprised... am I the only one who's more than a little disappointed at how poorly-documented ST is?

Wow. I'm glad that this exists, but uh... where the hell is this documented? Is it even?

Wow, uh.. okay. That works! Not really sure why that's not part of any of the standard themes, but.

Well the margin between the gutter and the text can now be changed with the "margin" setting in your preferences file.

I've found that you can simulate a gutter border by setting the "rulers" setting in your preferences to include column 0, e.g. { "rulers": [0] }. It's a little close to the text, but not bad.
+1, I was really surprised to find that the gutter color can't be changed separately from the text background.