ComboBox and DropDownList list items are wrapped in a div with an id which is created from the id of the html element from which the widget is initialized and the string "-list". In other words if we use <input id="countries" /> to initialize a ComboBox or a DropDownList the div containing the <ul> with the list items will have countries-list as an Id.
In order to achieve the desired result a CSS style could be applied to list items. Please refer to the example below:
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