I have tried to reproduce your scenario, but to no avail. Everything seems to work fine on my end.
Please ensure you that you have merged all necessary .xaml files for the chosen theme in the App.xaml file.
On the other hand, the SpellCheckingDialog is based on 'RadWindowStyle', too (the style is placed in Telerik.Windows.Controls.Navigation.xaml file).
If it will be more comfortable to you, you can send us a sample demo project of this issue and we will try to fix it.
If you have any further questions, please do not hesitate to contact us again.
the Telerik team