JSFiddle - React, Tailwind, and code Playground

HTML

<select id="bodyBtn" name="bodyBtn">
	<option>-- Выбрать --</option>
	<option value="#a1a1a1">#a1a1a1</option>
	<option value="#ff00ff">#ff00ff</option>
	<option value="#ff00ff">Выберите цвет</option>
</select>

JavaScript

var select = document.getElementById('bodyBtn');
select.onchange = function() {
     document.body.style.background = select.options[select.selectedIndex].value;
}