customize-mark-to-save is a compiled Lisp function in `custom.el'.

(customize-mark-to-save SYMBOL)

Mark SYMBOL for later saving.

If the default value of SYMBOL is different from the standard value,
set the `saved-value' property to a list whose car evaluates to the
default value. Otherwise, set it to nil.

To actually save the value, call `custom-save-all'.

Return non-nil if the `saved-value' property actually changed.