It's as a question, you ask: Did the player pressed a button? It is the button (Input::X)? It's a parameter to the method.
And, it's not Input.press?(), it's Input.trigger?().

To change the window skin, press help in the script editor. Search in the index for th class Window.

Don't need an update method for a window class, you can change the name and while the window is called in a class, you refresh this defined method. If you check in the sample, in Scene_Map, I only call the refresh method. I use more the update to update... you know? Like if you have a huge script that must refresh 10 differents methods. The update will gather all the method and refresh them.

(I took it from my karma script)
Like for a HUD:

Code:

#==============================================================================
# ** Karma_Hud
#------------------------------------------------------------------------------
#  This class handles the karma HUD.
#==============================================================================
class Karma_Hud < Window_Base
  include NuKaBuBbles_Karma
  #--------------------------------------------------------------------------
  # * Object Initialization
  #--------------------------------------------------------------------------
  def initialize
    super(HUDX, HUDY, 640, 64)
    self.contents = Bitmap.new(width - 32, height - 32)
    self.opacity = 0
    refresh
  end
  #--------------------------------------------------------------------------
  # * Refresh
  #--------------------------------------------------------------------------
  def refresh
    self.contents.clear
    self.contents.font.size = 24
    length = 1
    length = 0 if $game_party.karma.abs.to_s.size == 1
    length = ($game_party.karma.abs.to_s.size * 15) - (48 / 2) if $game_party.karma.to_s.size > 2
    if $game_party.karma >= Good
      draw_picture(-4, 2, 0, 0, 1, 1, Good_Pic)
      draw_numbers(16+48+length, 0, $game_party.karma, Number_Pic, 3, 0)
    elsif $game_party.karma <= Evil
      draw_picture(-4, 2, 0, 0, 1, 1, Evil_Pic)
      draw_numbers(16+48+length, 0, $game_party.karma, Number_Pic, 3, 2)
    else
      draw_picture(-4, 2, 0, 0, 1, 1, Neutral_Pic)
      draw_numbers(16+48+length, 0, $game_party.karma, Number_Pic, 3, 1)
    end
    if $game_switches[KPSWITCH] == true
      self.visible = true
    else
      self.visible = false
    end
  end
end

#==============================================================================
# ** Scene_Map
#------------------------------------------------------------------------------
#  This class performs map screen processing.
#==============================================================================
class Scene_Map
  include NuKaBuBbles_Karma
  #--------------------------------------------------------------------------
  # * Main
  #--------------------------------------------------------------------------
  alias karma_hud_main main
  def main
    @karma_hud = Karma_Hud.new
    karma_hud_main
    @karma_hud.dispose
  end
  #--------------------------------------------------------------------------
  # * Update
  #--------------------------------------------------------------------------
  alias karma_hud_update update
  def update
    karma_hud_update
    @karma_hud.refresh
  end
end

Ahhhhh, I never used the music variables in RGSS but, everything is possible. You must know that the limit, it's your knowledge.