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Gets or sets the ThemeColor value associated with this border.
Use this property to read or apply a workbook theme color to the border instead of a fixed Color. If the border does not use a theme color, this property returns None. When the workbook theme changes, the displayed border color may change accordingly.
ThemeColor ThemeColor { get; set; }
Property ThemeColor As ThemeColor
IBorder border = worksheet.Range["A1"].Borders[BordersIndex.EdgeBottom];
border.ThemeColor = ThemeColor.Accent1;
ThemeColor themeColor = border.ThemeColor;