Styling Editor Presenter
If you want to change the appearance of a cell when it enters edit mode, you should modify the style of the GridViewEditorPresenter. This element is a plain presenter used to display the editors for the cells.
To learn how to modify the default styles of a control, please refer to the Modifying Default Styles article.
You could also learn how to extract control templates in the following article.
Example 1: Styling the GridViewEditorPresenter
<Style TargetType="{x:Type telerik:GridViewEditorPresenter}">
<Setter Property="Template" Value="{StaticResource GridViewEditorPresenterTemplate}"/>
<Setter Property="Padding" Value="5"/>
</Style>
If you're using Implicit Styles, you should base your style on the GridViewEditorPresenterStyle.
Figure 2: RadGridView with styled GridViewEditorPresenter
If you would like to change the appearance of the editor itself, then you should create a simple style targeted at the corresponding editor. You can have a look at the Styling Column Editor article for more information.