This is a RadToggleButton which is present in the EditableTemplate of RadComboBox.
To apply any changes, you need the Expression dark theme resources for RadComboBox. You can obtain these from your RadControls installation folder, Themes folder.
The Style you need to edit is the EditableComboToggleButtonStyle
I hope this helps.
the Telerik team
Explore the entire Telerik portfolio by downloading the Ultimate Collection trial package. Get it now >>