I believe that the correct approach is to create your own custom theme and to modify the ValidationToolTip's style in an appropriate manner within ValidationTooltip.xaml file, please refer to the following help article.
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items