Hi Richard
Thank you for using Syncfusion Support.
We have checked the reported behavior related with SkinManager and observed that this type of warning error are not faced at our end. However the Theme property has been introduced in 18.3.0.35v , so when you use the previous versions of this version, then you will face this following error:
So we are unable to understand what might cause this error since it is working fine at our end with our lastly provided sample. So as requested , kindly ensure it by referring all nuget package are same of product version and also if possible kindly share your sample and share your currently using syncfusion product version that you are facing this warning error. It would be very helpful for us to trace the exact cause of the reported problem at our end. In order to avoid further delays, we can set up a web meeting session to check this error at your environment. So kindly let us know your availability, we would make every effort to schedule this meeting at your convenient time.
Please let us know if you need any other assistance on this.
Regards,
Vijayalakshmi VR