This property determines the appearance of solid marquee border that is drawn around the selected cell range. To set up the appearance of selected cells use Styles["Highlight"] style.

When SelectionMode is set to SelectionModeEnum.MultiRange only backround padding will be drawn.

To set the focus rectangle type use the FocusRect property. To set up the focused cell appearance use Styles["Focus"] style.

This property is themeable. Set needed values to subnodes of the c1theme template node to customize it.

By default marquee border is not shown. Turn it on with changing the C1.Win.FlexGrid.Util.Styles.MarqueeStyle.BorderMode property. It will draw 1px-width black solid line inside the selected range borders without padding for cell's background.

Set Office365 theme-like border.

var flexGrid = new C1.Win.FlexGrid.C1FlexGrid();
flexGrid.MarqueeStyle.BorderMode = MarqueeBorderMode.CellRange;

var flexGrid = new C1.Win.FlexGrid.C1FlexGrid();
flexGrid.MarqueeStyle.BackgroundPadding = new Padding(1);
flexGrid.MarqueeStyle.BackgroundPaddingColor = flexGrid.Styles.Normal.BackColor;
flexGrid.MarqueeStyle.BorderAlign = BorderAlign.Outside;
flexGrid.MarqueeStyle.BorderMode = MarqueeBorderMode.CellRange;
flexGrid.MarqueeStyle.Color = Color.Green;
flexGrid.MarqueeStyle.Width = 1;
flexGrid.FocusRect = FocusRectEnum.None

Reference

C1FlexGridBase Class
C1FlexGridBase Members