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private
  FDunkel : Boolean;
  function ChangeColor( Color : Integer; IsDark : Boolean ) : Integer;



function TForm1.ChangeColor( Color : Integer; IsDark : Boolean ) : Integer;
var r, g, b, a : Byte;
begin
  r := ( Color and $000000FF ) shr 0;
  g := ( Color and $0000FF00 ) shr 8;
  b := ( Color and $00FF0000 ) shr 16;
  a := ( Color and $FF000000 ) shr 24;

  if IsDark then
  begin
    r := r * 2;
    g := g * 2;
    b := b * 2;
  end
  else
  begin
    r := r div 2;
    g := g div 2;
    b := b div 2;
  end;

  result := Integer( r ) shl 0 + Integer( g ) shl 8 + Integer( b ) shl 16 + Integer( a ) shl 24;
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  I: Integer;
begin

  for I := 0 to self.ComponentCount - 1 do
  begin
    if Self.Components[ I ] is TPanel then
    begin
      TPanel( Self.Components[ I ] ).Color := ChangeColor( TPanel( Self.Components[ I ] ).Color, FDunkel );
      TPanel( Self.Components[ I ] ).Font.Color := ChangeColor( TPanel( Self.Components[ I ] ).Font.Color, FDunkel );
    end
    else if Self.Components[ I ] is TStringGrid then
    begin
      TStringGrid( Self.Components[ I ] ).FixedColor := ChangeColor( TStringGrid( Self.Components[ I ] ).FixedColor, FDunkel );
      TStringGrid( Self.Components[ I ] ).Color := ChangeColor( TStringGrid( Self.Components[ I ] ).Color, FDunkel );
      TStringGrid( Self.Components[ I ] ).Font.Color := ChangeColor( TStringGrid( Self.Components[ I ] ).Font.Color, FDunkel );
    end;
  end;

  Self.Color := ChangeColor( Self.Color, FDunkel );

  FDunkel := not FDunkel;

end;

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procedure TDimmForm.FormCreate(Sender: TObject);
begin
  AlphaBlend := true;
  AlphaBlendValue := 128;
  Color := clBlack;
  BorderStyle := bsNone;
  WindowState := wsMaximized;

  FormStyle := fsStayOnTop;
  SetWindowLong(Handle, GWL_EXSTYLE, getWindowLong(Handle, GWL_EXSTYLE) or WS_EX_TRANSPARENT);
end;

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function GetGrayLevel(const Color: TColor): Integer;
begin
  Result := (77 * (Color and $FF) + 151 * (Color shr 8 and $FF) + 28 * Color shr 16 and $FF) shr 8;
end;

function GetGoodContrast(const Color: TColor): TColor;
begin
  if GetGrayLevel(Color) < 128 then Result := clWhite
  else Result := clBlack;
end;

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Form1.Color := clBlack; //DeineNeueFarbe
Form1.Font.Color := GetGoodContrast(Form1.Color);