Base on your code, I search for every "new TextBox" (found 3) and give it a specific font as below (Red color)
txt = new TextBox();
txt.FontFamily = new FontFamily("Tahoma");
txt.Height = 20;
txt.BorderThickness = new Thickness(0);
This would work very well for my requirement for now. (my machine has this font already)
However, to get it work on any machine. I think I will have to work more on embedded font.
Seems that your suggestion can change default font of the application. Am I right?
I will study it. Thanks a lot for your kindness.