1. See the Control5 DropdownList example. Selection sends a controlEvent with the value (which can be 0, 1, 2, etc.). You can store the last selection in a variable, if you want to look it up at a later time or you can make something happen directly from the controlEvent method.

2. You can change the font of the valuelabel, but I'm not sure if you can set the font for individual listbox items. The easiest way would be to change the global controlP5 font. This will change all fonts (also of the complete dropdownlist):

1.   cp5.setControlFont(new ControlFont(createFont("Arial", 20), 20));

3. Yes, with something like: .setColorBackground(color(0,0,255))

4. Not as far as I know. But you can get the text, change the text and set the text to remove unwanted characters. For example you could make it a letters-only or digits-only textfield. See the example below:

1. void remover() {
2.   String txt = cp5.get(Textfield.class,"input").getText();
3.   txt = txt.replaceAll("\\P{L}", ""); // letters only
4. //  txt = txt.replaceAll("\\D+", ""); // digits only
5.   cp5.get(Textfield.class,"input").setText(txt);
6. }