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