Each content panel has two properties called BorderStyle and BackgroundFillStyle that allow you to set a border and fill style applied on the panel background. By default both are set to null indicating that the panel does not have a background and border. The following picture shows a numeric display panel with applied round border and black background:

In order to change the border style you need to create an instance of one of the following types:

The following code shows how to apply a border and background fill style to a panel:

NNumericDisplayPanel numericDisplay = new NNumericDisplayPanel();
numericDisplay.BorderStyle = new NEdgeBorderStyle();
numericDisplay.BackgroundFillStyle = new NColorFillStyle(Color.LightGray);
numericDisplay.Value = 123456.789;
nChartControl1.Panels.Add(numericDisplay);

NNumericDisplayPanel numericDisplay = new NNumericDisplayPanel();
numericDisplay.BorderStyle = new NEdgeBorderStyle();
numericDisplay.BackgroundFillStyle = new NColorFillStyle(Color.LightGray);
numericDisplay.Value = 123456.789;
nChartControl1.Panels.Add(numericDisplay);