You can define a custom palette that inverts the default colors, like this:
C#:
// declare a custom palette
var palette = new TerminalPalette();
for (int i = 0; i < 16; i++)
{
Color c = palette.GetColor(i);
c = Color.FromArgb(255 - c.R, 255 - c.G, 255 - c.B);
palette.SetColor(i, c);
}
// assign the palette to TerminalControl object
terminalControl.Palette = palette;