uiTheme will return only two color #FF000000(Dark)or #FFFFFFFF (Light) according to system ui theme, so you can go this way.
you can fill dark or light color to your some ui element to show current system theme
var DefaultTheme = new Windows.UI.ViewManagement.UISettings();
var uiTheme = DefaultTheme.GetColorValue(Windows.UI.ViewManagement.UIColorType.Background).ToString();
if(uiTheme == "#FF000000")
{
WindowsThemeText.Text = "Dark";
}
else if(uiTheme == "#FFFFFFFF")
{
WindowsThemeText.Text = "Light";
}
/*
else
{
WindowsThemeText.Text = "Some new Updated theme found";
}
*/