JSFiddle - React, Tailwind, and code Playground

HTML

<textarea type="textarea" class="formArea" name="output" onclick="this.focus();this.select()" 
id="output" cols="70" rows="10" placeholder="Some text"></textarea>
<br>
<div class="btn-group">
    <button value="Combine" class="buttonGroup" onclick="convert()" id="combine1">
        Button
    </button><br><br>
</div>
<br>
<textarea type="textarea" class="formArea" name="output" onclick="this.focus();this.select()" 
id="output" cols="70" rows="5" placeholder="Some text"></textarea>
<br>
<div class="btn-group">
    <button value="Combine" class="buttonGroup" onclick="convert()" id="combine1">
        Button
    </button><br><br>
</div>
<br>
    <ul id="switcher">
  <li id="blackButton">

CSS

body {
    background-color: #ec7c23;
    }
    .btn-group button {
    background-color: #577CC1; 
    border: 1px #577CC1; 
    color: white; 
    padding: 10px 24px; 
    cursor: pointer; 
    float: left; 
    font-weight: bold;
    }
    .btn-group:after {
    content: "";
    clear: both;
    display: table;
    }
    .btn-group button:not(:last-child) {
    border-right: none;
    }
    .btn-group button:hover {
    background-color: #2C3E60;
    }


    textarea[type="textarea"] {
    width: ;
    padding: 5px 10px;
    margin: 5px 0;
    box-sizing: border-box;
    font-family: arial;
    font-weight: bold;  
    border: 10px;
    background-color: #F4F6FA;
    color: darkblue; 
    border: 4px solid #577CC1;
    border-radius: 4px;
    }
    textarea[type="textarea"]:focus {
    border: 4px solid #e66026;
    }


    h1 {
    line-height: 1.25;
    margin: 2em 0 0;
    }
    p {
    margin: .5em 0;
    }
    #switcher {
    list-style: none;
    margin: 0;
    padding: 0;
    overflow: hidden;
    }
    #switcher li {
    float: left;
    width: 30px;
    height: 30px;
    margin: 0 15px 15px 0;
    border-radius: 30px;
    border: 3px solid black;
    }
    #blackButton {
    background: red;
    }

JavaScript

document.getElementById('blackButton').onclick = switchBlack;


    function switchBlack() {

    var body = document.getElementsByTagName('body')
    for(var i = 0; i < body.length; i++) {
    body[i].style.color = 'white';
    body[i].style.backgroundColor = "grey";
    body[i].style.fontFamily = 'terminal';
    }
    
    var formArea = document.getElementsByClassName('formArea')
    for(var i = 0; i < formArea.length; i++) {
    formArea[i].style.backgroundColor = 'black';
    formArea[i].style.borderColor = 'white';
    formArea[i].style.color = 'white';
    formArea[i].style.fontFamily = 'terminal';
    }
    


    var css = '.btn-group button {background-color: purple; color:black; border-color: white; font-       family: terminal; font-weight: bold; border-radius: 0px;}';
    var style = document.createElement('style');
    if (style.styleSheet) {
    style.styleSheet.cssText = css;} 
    else {
    style.appendChild(document.createTextNode(css));
    }
    document.getElementsByTagName('head')[0].appendChild(style);
  
    var css = '.btn-group button:hover {background-color: red; color: white;}';
    var style = document.createElement('style');
    if (style.styleSheet) {
    style.styleSheet.cssText = css;}
    else {
    style.appendChild(document.createTextNode(css));
    }

    document.getElementsByTagName('head')[0].appendChild(style);
 
    }