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`custom-theme-load-path' was a defcustom, but it shouldn't be for the
same reason that `load-path' shouldn't.  Setting it via the customize
interface is a trap for the user.

Installed themes commonly add themselves to this variable, which means
its value is not fit for being saved (it will permanently remember dirs
that don't exist anymore).

This is aggravated by the fact that Emacs always applies the `user'
theme on top of any theme that's loaded, since this will apply the old
variable value and remove any new directories that had been recently
added by themes themselves.

Not to mention, we already have `custom-theme-directory', which is safe
to customize.