Hello Shaikh Ahmad,
Font colors come from the JetPack theme - for example, the foreground color for tab pages came from the TextBlock style in CoreStyles.xaml, which in turns takes its value from NormalFontBrush brush from Brushes.xaml. If you edit these values, you can eventually achieve what you want. For example, this edit in Brushes.xaml from the JetPack theme will make fore color for most of the default controls black:
All the best,
Boby
the Telerik team
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