Couldn't switch from classic to modern theme, using edit... preferences...
I have the sneaking suspicion that this is a bug in the size of the prefs panel.
If you increase the size of the panel the "Apply Theme" button will show up and
you should be able to click it.
Yup, that was it. I went ahead and changed the summary.
This also affects the following preferences screens:
Smart Browsing: Enable Location bar autocomplete checkbox.
Mail & Newsgroups
Message Display: Apply default to all messages checkbox.
All of the information is displayed once the screen size is
adjusted, but this might not be common knowledge to a novice
Fixed in 0.9.1 which should show up in rawhide shortly.