Hi Yash
Thank you for contacting Syncfusion Support.
As per WPF framework, we can able to style the appearance , only if control have Template property. So only we can able to provide the styling and themes for particular control. Since the MessageBox does not have these properties , the themes are not applicable for this control.
Please refer the following link for more information:
Please get back us, if you any further queries.
Regards,
Vijayalakshmi VR