JSFiddle - React, Tailwind, and code Playground

HTML

<label for="theme-grey"><input type="radio" id="theme-grey" name="theme" value="grey" checked />Grey</label>
<label for="theme-pink"><input type="radio" id="theme-pink" name="theme" value="pink" />Pink</label>
<label for="theme-green"><input type="radio" id="theme-green" name="theme" value="green"/>Green</label>

<button id="check">Check</button>

JavaScript

document.getElementById('check').addEventListener('click',function(){
    var value = document.querySelector('[name="theme"]:checked').value;
    alert(value);    
});