to change the tool tip text for the Minimize button to "Closed" you have to edit the control template of the RadTileViewItem control in Blend. For more information you can follow the link below: http://www.telerik.com/help/silverlight/radcontrols-for-silverlight-expression-blend-support.html
Please find attached a project with a custom theme and changed tool tip text.
If you have further questions please do not hesitate to ask us.
I hope this will help you.
the Telerik team
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