DrawContent

 public class MessageBubbleEditPainter : RichTextEditPainter
    {


        protected override void DrawContent(ControlGraphicsInfoArgs info)
        {
            var darkBlueColor = Color.FromArgb(18, 61, 87);
            var blueBrush = new SolidBrush(darkBlueColor);
            var bluePen = new Pen(darkBlueColor);
            var backColor = info.ViewInfo.Appearance.GetBackColor();
            if (backColor == Color.Empty) backColor = Color.White;
            if (info.ViewInfo is MessageBubbleEditViewInfo vi)
            {
                vi.FillBackground = true;
                info.ViewInfo.PaintAppearance.BackColor = darkBlueColor;
                info.ViewInfo.PaintAppearance.ForeColor = Color.White;
                Brush brush = new SolidBrush(backColor);
                info.Graphics.SmoothingMode = SmoothingMode.AntiAlias;
                info.Graphics.FillRectangle(brush, info.Bounds);
                var rect = vi.ClientRect;
                rect.Inflate(4, 4);
                info.Graphics.DrawRectangle(bluePen, rect);
                info.Graphics.FillRectangle(blueBrush, rect);

                var bubbleTail = PlacementHelper.AlignTriangleToRectangle(rect, ContentAlignment.MiddleLeft, 3, 6);
                info.Graphics.FillPolygon(blueBrush, bubbleTail);
            }
            base.DrawContent(info);
        }
    }