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The reported issue may occur due to the unavailability of Theme.Base assembly reference in your application. We had introduced a new assembly called “Syncfusion.Theme.Base” in our Volume 3, 2012 release (v10.3.0.43) for rich theming support. We need to refer that assembly in web.config and application reference as well, while hosting the application to server. Whereas while running the application from local, it will load the Theme.Base assembly from GAC directly (if it is in GAC). If it is not in GAC, we need to refer it manually from BIN directory or any other local directorty.
Please refer the below code snippet,
<add assembly="Syncfusion.Theme.Base, Version=10.404.0.71, Culture=neutral, PublicKeyToken=3d67ed1f87d44c89"/> //replace the version based on your Essential studio version
Kindly try the above suggestion and let us know if it helps.