GUI.Textfield backspace control + Button Style problem

Hi, i've made a system of button input like the Windows calculator. I don't know how to setup the backspace when pushing the button.

 if(GUI.Button(Rect (xdist+25+5, yoffset+21+ydist+21+ydist+21+ydist, 55, 21), "All", "All")){
     stringToEdit = itemQuantity.ToString();
     UI.click = true;        
 }
 GUI.Label(Rect (xdist+25+5+20, yoffset+21+ydist+21+ydist+21+ydist+2, 50, 50), "All");
 if(GUI.Button(Rect (xdist+25+5+25+5+25+5, yoffset, 26, 50), "B/S", "Lateral")){
     //here, i would like to put the backspace function
     UI.click = true;        
 }
 GUI.Label(Rect (xdist+25+5+25+5+25+5+2, yoffset+17, 26, 50), "B/S");
 if(GUI.Button(Rect (xdist+25+5+25+5+25+5, yoffset+21+ydist+21+ydist, 26, 50), "C", "Lateral")){
     stringToEdit = "0";
     UI.click = true;        
 }
 GUI.Label(Rect (xdist+25+5+25+5+25+5+8, yoffset+21+ydist+21+ydist+17, 26, 50), "C");

Then, i have an other problem. The button text function works only with the default button style. When i create a custom style, i setup it exactly like the default one, but no text seems to appear in my buttons.

If you have any idea, thanks.