If you begin entering e.g. "=" in FormulaBar, the characters are not visible in dark themes. If the focus is in the cell it works as expected, if the focus is in FormulaBar, the Foreground is dark and cannot be seen on dark background. 

Workaround:
-  set the following style (add BaseOn if using implicit styles)

<Style  TargetType="spreadsheetControls:RadSpreadsheetFormulaBar" BasedOn="{StaticResource RadSpreadsheetFormulaBarStyle}">
            <Setter Property="Foreground" Value="White"/>
</Style>

Available in LIB Version 2017.2.828.