This is one of those things I’m going to forget and it’s drop-dead simple. So for others new to Drupal and before it slips my mind….
Go straight to “Administer” / “Administration theme” and change the theme in the dropdown from “System default” to a theme that you know is stable. With version 5, the best choice is “Garland.”
Then one day when you make some hideous mistake in the custom theme you are developing and it blows up your ability to get to much of anything, your administration panel is safe.
For the rest of the time, your home page and content areas, even when you are signed in as an admin, have the (hopefully not hideous) look-and-feel you are developing.