window's initialize method
class Window_Message (or whatever window)
def initialize
# Everything already in the method
self.back_opacity = xxx
end
end
Or, as ahref said, in the Scene class you want to modify, right after you make the window, so if in Scene_Menu or something:
def main
# Bla bla bla
@menustatus_window = Whatever
@menustatus_window.opacity = xxx
# Bla bla bla
end