JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://code.jquery.com/jquery-1.11.0.min.js"></script>
<div id="koRoot">
    <div class="myClass">
        <input type='text' data-bind="value: customColor" />
    </div>
    <style data-bind="text: myObservableStyle(customColor)"></style>
</div>

CSS

.myClass { background-color: #f00 }

JavaScript

$(document).ready(function () {
    var ViewModel = {
        customColor: ko.observable("#f00"),
        myObservableStyle: function (obs) {
            return ".myClass { background-color: " + obs() + " }";
        }
    }
    ko.applyBindings(ViewModel, document.getElementById("koRoot"));
});