JSFiddle - React, Tailwind, and code Playground

HTML

<input type="checkbox" name="locationthemes" id="checkbox-1" value="2" class="custom" checked />
<label for="checkbox-1">Castle</label>
<input type="checkbox" name="locationthemes" id="checkbox-2" value="3" class="custom" />
<label for="checkbox-2">Barn</label>
<input type="checkbox" name="locationthemes" id="checkbox-3" value="5" class="custom" checked />
<label for="checkbox-3">Restaurant</label>
<input type="checkbox" name="locationthemes" id="checkbox-4" value="8" class="custom" />
<label for="checkbox-4">Bar</label>

JavaScript

[].forEach.call(document.querySelectorAll('input[name="locationthemes"]:checked'), function(cb) {
   console.log(cb.value); 
});