Hi Soon,
Thanks for contacting Syncfusion.
In order to override theme from package you should override specific Style key provided in package.
Instead of BasedOn="{StaticResource {x:Type Button}}" ,
please use
BasedOn="{StaticResource MetroButtonStyle}"
Here, MetroButtonStyle is the key provided in Source. The naming convention of Style key is,
Themename+ControlName+ Style. For example MetroDateTimeEditStyle.
We have created a simple sample for this, and please have it from following location
Sample link:
SkinSample_119772
Please let us know in case of any further assistance.
Regards,
Muthukumar A