Hello Crepet Stephane,
With Q1 2010 we introduced a major facelift for all 6 built-in themes in order to deliver a more consistent look and feel for all Silverlight controls. This was triggered after a lot of feedback concerning inconsistencies in the visual appearance among our controls. It is not possible to use the old default theme. The only way is to create a custom theme and make it look like the old default one.
Sorry for the inconvenience caused.
the Telerik team
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