C#

void Panel1_Paint(object sender, PaintEventArgs e)
{

    Graphics Graphics = e.Graphics;
    Rectangle Bounds = new Rectangle(Point.Empty, splitContainer1.Panel1.Size);

    if (Bounds.Width > 0 && Bounds.Height > 0)
    {

        using (Brush GradientBrush = new LinearGradientBrush(Bounds, Color.FromArgb
        (183, 214, 183), Color.FromArgb(221, 235, 221),
        LinearGradientMode.BackwardDiagonal))
        {
            Graphics.FillRectangle(GradientBrush, Bounds);
        }
    }
}

private void Form1_Load(object sender, EventArgs e)
{
    splitContainer1.Panel1.Paint += new PaintEventHandler(Panel1_Paint);
}

Client client = new Client();