It looks like the `Fluent` themes may not properly theme the Non-Editable RadComboBox control.
Within the default style for RadComboBox within Telerik.Windows.Controls.Input.xaml, there exists a RadToggleButton with the name "PART_DropDownButton" which is part of the main control template. This toggle button uses the style "NonEditableComboToggleButtonStyle", which then sets the toggle button template to the "NonEditableComboToggleButtonControlTemplate" control template.
This control template has a Border named "ToggleButtonBackground" where to background is set to a template binding. However, the NonEditableComboToggleButtonStyle does not set background, and it therefore leads to a control that looks like the attached image.
This can be fixed by overriding the RadComboBox style, but it isn't simple to set the background on the ToggleButton, so you need to create custom templates and style to do it.