The instance of the Preferences structure in memory can be changed with
the Intuition SetPrefs() function:

    struct Preferences *SetPrefs(struct Preferences *preferences,
                                 LONG size, BOOL inform);

In addition to a buffer holding the Preferences structure, and the buffer
size, this function takes an argument which indicates whether an
IDCMP_NEWPREFS message should be broadcast to windows which have this flag
set in the Window.IDCMPFlags field of their window.

    Avoid Using SetPrefs().
    -----------------------
    This function is normally only used by Preferences-like utilities.
    There should be no need for a normal application to set the system
    Preferences with SetPrefs().