Write the "hidden" attribute of a style into the collection's attribute when using the theme editor. Otherwise, the next time using the theme editor, the hidden attributes that should not be changed by the user are visible.

SYMBOLSET
SYMBOL
  NAME 'point'
  TYPE ELLIPSE
  POINTS
    1 1
  END
  FILLED TRUE
END
SYMBOL
  NAME "square"
  TYPE VECTOR
  POINTS
    0 0
    0 1
    1 1
    1 0
    0 0
  END
  FILLED TRUE
END
END