Hi,

I think the following patch does the trick.  Here is what I tested with
it:

        - A call to `clean-buffer-list' cleans up the list once and does
          not activate the midnight-timer.

        - The midnight mode is toggled on/off by calling
          `midnight-mode'.

        - Calling (customize-variable 'midnight-delay) stops and
          restarts the midnight-timer with the updated delay.

Maybe the line ":initialize #'custom-initialize-default" could be
removed, I don't know.  WDYT?