CSS применима к выбранной опции в теге Select
<form id="location-options">
<select name="location-category" id="location-category">
<option value="">Select</option>
<option value="test1">Imbiss</option>
<option value="test2" selected>Restaurant</option>
<option value="test1">Imbiss</option>
<option value="test2">Restaurant</option>
</select>
</form>
<style>
#location-options select option[selected],
#location-options select option:hover,
#location-options select option:focus,
#location-options select option:active,
#location-options select option:checked {
background: red;
color: white
}
</style>
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Lokesh Thakur