Код: Выделить всё
public FlatButton() : base()
{
FlatStyle = System.Windows.Forms.FlatStyle.Flat;
FlatAppearance.BorderSize = 0;
FlatAppearance.BorderColor = Color.FromArgb(0, 255, 255, 255);
BackColor = Color.FromArgb(0, 255, 255, 255);
}
protected override void OnPaint(PaintEventArgs e)
{
base.OnPaint(e);
Pen pen = new Pen(FlatAppearance.BorderColor, 0);
Rectangle rectangle = new Rectangle(0, 0, Size.Width - 2, Size.Height - 2);
e.Graphics.DrawRectangle(pen, rectangle);
}


Подробнее здесь: https://stackoverflow.com/questions/367 ... ent-border