If border_style is Fixed3D, the code checks the sunken parameter and initializes an array to the system colors it should use for the different parts of the border. It then uses the Graphics object's DrawLine method to draw the border.

' Draw a border inside this rectangle.
Private Sub DrawBorder(ByVal gr As Graphics, ByVal rect As _
    Rectangle, ByVal border_style As BorderStyle, Optional _
    ByVal sunken As Boolean = True)
    Select Case border_style
        Case BorderStyle.FixedSingle
            rect.Width -= 1
            rect.Height -= 1
            gr.DrawRectangle(Pens.Black, rect)
        Case BorderStyle.Fixed3D
            Dim colors() As Color
            If sunken Then
                colors = New Color() { _
                    SystemColors.ControlDark, _
                    SystemColors.ControlDarkDark, _
                    SystemColors.ControlLightLight, _
                    SystemColors.ControlLight _
                }
            Else
                colors = New Color() { _
                    SystemColors.ControlLightLight, _
                    SystemColors.ControlLight, _
                    SystemColors.ControlDark, _
                    SystemColors.ControlDarkDark _
                }
            End If
            Dim p As Pen
            p = New Pen(colors(0))
            gr.DrawLine(p, rect.X, rect.Y + rect.Height - _
                1, rect.X, rect.Y)
            gr.DrawLine(p, rect.X, rect.Y, rect.X + _
                rect.Width - 1, rect.Y)
            p = New Pen(colors(1))
            gr.DrawLine(p, rect.X + 1, rect.Y + rect.Height _
                - 2, rect.X + 1, rect.Y + 1)
            gr.DrawLine(p, rect.X + 1, rect.Y + 1, rect.X + _
                rect.Width - 2, rect.Y + 1)
            p = New Pen(colors(2))
            gr.DrawLine(p, rect.X, rect.Y + rect.Height - _
                1, rect.X + rect.Width - 1, rect.Y + _
                rect.Height - 1)
            gr.DrawLine(p, rect.X + rect.Width - 1, rect.Y _
                + rect.Height - 1, rect.X + rect.Width - 1, _
                rect.Y)
            p = New Pen(colors(3))
            gr.DrawLine(p, rect.X + 1, rect.Y + rect.Height _
                - 2, rect.X + rect.Width - 2, rect.Y + _
                rect.Height - 2)
            gr.DrawLine(p, rect.X + rect.Width - 2, rect.Y _
                + rect.Height - 2, rect.X + rect.Width - 2, _
                rect.Y + 1)
    End Select
End Sub