ChartSurface.ExportToFile() after changing theme

hi,
I am using ExpressionDark theme for my scichart surface.
I want to do ExportToFile after changing theme to BrightSpark.
When I do exporttofile after change the theme to BrightSpark, my exported image is still in ExpressionDark.

I do it like below.

ThemeManager.SetTheme(Chart, “BrightSpark”);
var tempFileName = Path.GetTempFileName();
Chart.ExportToFile(tempFileName, ExportType.Png);
var i = Image.FromFile(tempFileName);

Can some one help please?