This article points to the RadComboBox API that can be used to customize its appearance using styles and templates.
To customize the appearance of the items when RadComboBox's ItemsSource is set, use the ItemTemplate property. An example can be found in the Binding to Collection article.
Additionally, you can use the ItemTemplateSelector property to implement a DataTemplateSelector for the items.
RadComboBox shows a text input area when in edit mode (IsEditable=True) represented by a TextBox control. The control can be customized via the TextBoxStyle property of RadComboBox. Read more, in the TextBoxStyle article.
The Selection Box part of RadComboBox is customized via the SelectionBoxTemplate, MultipleSelectionBoxTemplate and EmptySelectionBoxTemplate properties. Read more, in the Selection Box Template article.
You can edit the control template of RadComboBox in order to achieve visualization and functionality that is not provided out of the box or via the built-in API. To do this, extract the ControlTemplate of the control and modify the elements in it. Read more about extracting templates in the Editing Control Templates article.
To apply the customized ControlTemplate, use the NonEditableTemplate and EditableTemplate properties of RadComboBox. NonEditableTemplate is applied when the IsEditable property of the control is set to False (default). EditableTemplate is applied when IsEditable is True.
All elements in the ControlTemplate that contains "PART_" in their Name should be present in the template. Those are required elements used in code by the control. Removing the required elements will lead to missing functionality or runtime errors.