Add an option to assign a Style for the ScreenTip control. The option should be available per RadRibbonView and also per each separate element. It could follow the current ScreenTip settings concept where attached properties are used.

For example:

<telerik:RadRibbonView telerik:ScreenTip.Style="{StaticResource MyGlobalScreenTipStyle}" />

or:

<telerik:RadRibbonButton telerik:ScreenTip.Style="{StaticResource MyButtonScreenTipStyle}" />