Your problem is occurring because you are modifying the aesthetics of your buttons in the wrong place.
btnA100_Click method is executed only when clicking on a button (I assume
If what you are looking for is to paint at startup, you can do it using either the constructor of your form, or the event
Load of it.
public partial class Form1 : Form
//Acá podes pintarlo de la manera que lo haces.
Or also, as I mentioned above, you can subscribe the form to the event
Load , for example
this.Load += Form1_Load1; //Asumiendo que this referencia a tu Form
and then in what you paint in the method associated with the event
private void Form1_Load1(object sender, EventArgs e)
//Aca podes pintarlo de la manera que lo haces.
I hope I have clarified your doubts.