So on my installation of Gnome Inform (6.33/6L38), this is what the Skein looks like. Ie, kind of unreadable. Is this a bug, some issue with GTK/Gnome configuration, or just what it’s supposed to look like?
It’s not supposed to look like that.
…I don’t know any more than that. Only that it’s not. It’s supposed to look like tech.thingoid.com/assets/inform7-ide-skein.png
Well, that’s what it looks like under Windows anyway. The different IDEs are different, so I was unsure.
You mean I was even less helpful than I thought? Good gracious. 
I’m running the GNOME IDE and I can tell you that my skein looks correct and readable (ie, not like yours). So I can tell you that yours is definitely broken, but sadly I can’t tell you why.
What distribution and GTK theme are you using? It might just be an issue with the theme engine.
I’m running Linux Mint with the Xfce desktop, so technically not GNOME. But the GNOME IDE runs fine in it.