TraditionalForm[Style[Framed[
!(*OverscriptBox[(X), (_)]) == HoldForm[1/n]*!(
*UnderoverscriptBox[([Sum]), (i = 1), (n)]
*SubscriptBox[(X), (i)]), FrameMargins -> 15], FontFamily -> #,
    FontSize -> 24, Bold]] & /@ {"Tahoma", "TimesNewRoman",
  "Helvetica"}

Ac[expr_] :=
  DynamicModule[{c1 = 0},
                  EventHandler[
                   Dynamic[
                                        If[c1 == 0,

      Style[expr, Black, Plain, 22, Italic] // ExpressionCell,

      Dynamic@If[Clock[1, 0.7, 2] < .5,

         Style[expr, Gray, Plain, 22, Italic] // ExpressionCell,

         Style[expr, RGBColor[0.8, 0, 0], Bold, 23, Italic]] //
       ExpressionCell
                                             ]
                                  ],

    {"MouseDown" :> (c1 = c1 /. {0 -> 1, 1 -> 0})}
                                          ]
                    ];

Pl[expr_] := ExpressionCell[Style[expr, Black, Plain, 22, Italic]];

Panel@Row[{"f=[Integral]{" // Pl, " y(x)" // Ac, " + " // Pl,
   "z(x)" // Ac, "}dx         (1)" // Pl}]