You set the legend background fill through the BackgroundFill property - the following code snippet modifes the legend background fill to semitransparent white:

legend.BackgroundFill = new NColorFill(NColor.FromColor(NColor.White, 0.5f));

The legend grid lines visually separate the data items horizontally and vertically. You control the appearance of horizontal and vertical grid lines through the HorizontalGridStroke and VerticalGridStroke properties. The following code shows how to change the horizontal grid lines to thick solid black:

legend.HorizontalGridStroke = new NStroke(2, NColor.Black);

The legend interlace stripes are automatically generated based on the interlace styles added to the InterlaceStyles collection of the legend. Each interlace style is represented by a NLegendInterlaceStyle object controlling the interlace stripes appearance and orientation (row / column). Furthermore, the interlace stripes have Z order - stripes generated by interlace styles on top of the collection appear below interlace stripes generated by the styles afterward. The following code creates a row and column interlace style:

NLegendInterlaceStylesCollection interlaceStyles = new NLegendInterlaceStylesCollection();
NLegendInterlaceStyle horzInterlaceStyle = new NLegendInterlaceStyle();
horzInterlaceStyle.Fill = new NColorFill(NColor.FromColor(NColor.LightBlue, 0.5f));
horzInterlaceStyle.Type = ENLegendInterlaceStyleType.Row;
horzInterlaceStyle.Length = 1;
horzInterlaceStyle.Interval = 1;
interlaceStyles.Add(horzInterlaceStyle);
NLegendInterlaceStyle vertInterlaceStyle = new NLegendInterlaceStyle();
vertInterlaceStyle.Fill = new NColorFill(NColor.FromColor(NColor.DarkGray, 0.5f));
vertInterlaceStyle.Type = ENLegendInterlaceStyleType.Col;
vertInterlaceStyle.Length = 1;
vertInterlaceStyle.Interval = 1;
interlaceStyles.Add(vertInterlaceStyle);
legend.InterlaceStyles = interlaceStyles;