Good. Now the admin is near self-contained (images use url() addressing), and will be in default style. And you can hack away at the upper level templates and stylesheet as you wish (without crippling admin). This is not a complete defence, there are a few other links, and don't, ever delete JForum supplied images, or the admin pages will go for a walk.
Still, glad I figured this when I did (a couple of hours ago).
I pushed this a little and found table styling does mess the admin display. I figure table styling is very likely, so here's revised instructions,
1 (as above) Use a clean default styles.css sheet, put it in a subfolder of admin, and point the main header at it.
2 Also, point (both in /admin) menu.htm and admin_welcome.htm at the /admin/style.css.
And that should do it. I suppose you could refactor completely, for complete sandboxing (of an admin theme), but the above should cover most theme styling activity.