Custom javascript button to change body background and font color

Note: not a duplicated because this is a custom made code and no other answer is answering my question, also because this is only javascript and not Jquery based

After reading hundreds of other posts and answers I decided to create a simple function instead of complicated/advanced ones, using just javascript. The problem is that I just started to learn javascript yesterday

What I want:

I want users to click on a single toggle button to turn dark mode on/off without complications in the code but just to save the value in localStorage and/or retrieve the chosen values from localStorage

The colors are just 2:

Dark mode is: body background #101010 and body font color if #FFFFFF

Light mode is: body background #FFFFFF and body font color if #101010

What I have

HTML

<button class="change" value="on">Dark mode is on</a>
<button class="change" value="off">Dark mode is off</a>
#would be nice with a single toggle button

Javascript

var document.querySelector(".change")
var body = document.getElementsByTagName("body")

btn.addEventListener("click", (e) => {
if (body[0].style.backgroundColor === "on") {
body[0].style.backgroundColor = "#101010"
body[0].style.color = "#FAFAFA"

} else {
body[0].style.backgroundColor = "#FAFAFA"
body[0].style.color = "#101010"

}
localStorage.setItem("bgColor", body[0].style.backgroundColor)
localStorage.setItem("textColor", body[0].style.color)
})
body[0].style.backgroundColor = localStorage.getItem("bgColor")
body[0].style.color = localStorage.getItem("textColor")

What I need

To make it save the chosen colors by the user and remember them with localstorage

To know if it is possible to make the button toggle with the text:
Dark mode is on/Dark mode is off

I created this Codepen with the code
https://codepen.io/familias/pen/gOQMrjX

This should work fine now, I just added on to your code.

The selected (mode)colors by the user will be saved in localStorage and retrieved when the page is loaded. Additionally I updated the CSS to include a .dark-mode class that applies the dark mode styles to the body element.

html
<button id="toggleBtn">Dark mode is on</button>

javascript

var toggleBtn = document.getElementById("toggleBtn");
var body = document.getElementsByTagName("body")[0];

toggleBtn.addEventListener("click", function() {
  var bgColor, textColor;

  if (body.classList.contains("dark-mode")) {
    body.classList.remove("dark-mode");
    toggleBtn.textContent = "Dark mode is on";
    bgColor = "#FFFFFF";
    textColor = "#101010";
  } else {
    body.classList.add("dark-mode");
    toggleBtn.textContent = "Dark mode is off";
    bgColor = "#101010";
    textColor = "#FFFFFF";
  }

  body.style.backgroundColor = bgColor;
  body.style.color = textColor;

  // Save the chosen colors to localStorage
  localStorage.setItem("bgColor", bgColor);
  localStorage.setItem("textColor", textColor);
});

// Retrieve saved colors from localStorage
var savedBgColor = localStorage.getItem("bgColor");
var savedTextColor = localStorage.getItem("textColor");

if (savedBgColor && savedTextColor) {
  body.style.backgroundColor = savedBgColor;
  body.style.color = savedTextColor;

  if (savedBgColor === "#101010" && savedTextColor === "#FFFFFF") {
    body.classList.add("dark-mode");
    toggleBtn.textContent = "Dark mode is off";
  }
}

css
body {
  margin: 0;
  height: 100%;
}

.dark-mode {
  background-color: #101010;
  color: #FFFFFF;
}

Newb here also, buy I'll try to help you.

I suggest using a checkbox. You can use CSS to make a slider button. Follow below what you need with my comments.

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-css -->

/* The switch - the box around the slider */
.switch {
position: relative;
display: inline-block;
width: 60px;
height: 34px;
}
/* Hide default HTML checkbox */
.switch input {
opacity: 0;
width: 0;
height: 0;
}
/* The slider */
.slider {
position: absolute;
cursor: pointer;
top: 0;
left: 0;
right: 0;
bottom: 0;
background-color: #ccc;
-webkit-transition: .4s;
transition: .4s;
}
.slider:before {
position: absolute;
content: "";
height: 26px;
width: 26px;
left: 4px;
bottom: 4px;
background-color: white;
-webkit-transition: .4s;
transition: .4s;
}
input:checked + .slider {
background-color: #2196F3;
}
input:focus + .slider {
box-shadow: 0 0 1px #2196F3;
}

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

window.addEventListener('DOMContentLoaded', function() {
var checkbox = document.getElementById('change'); //Get the status os the checkbox
checkbox.addEventListener('change', function() { //Checks if the status changed
if (checkbox.checked) {
document.body.classList.add('dark-theme'); //Uses the dark-theme that is in the CSS file.
localStorage.setItem('theme', 'dark'); //Saves the "theme" in local storage
} else {
document.body.classList.remove('dark-theme'); //Goes back to normal model in the CSS file
localStorage.setItem('theme', 'light');  //Saves the "theme" in local storage
}
});
var savedTheme = localStorage.getItem('theme'); //When you load the page, it will check which 'theme' is stored. If it is normal, it does nothing, if it's dark, checks the checkbox and uses the dark-theme in CSS.
if (savedTheme === 'dark') {
checkbox.checked = true;
document.body.classList.add('dark-theme');
}
});

<!-- language: lang-css -->

/* The switch - the box around the slider */
.switch {
position: relative;
display: inline-block;
width: 60px;
height: 34px;
}
/* Hide default HTML checkbox */
.switch input {
opacity: 0;
width: 0;
height: 0;
}
/* The slider */
.slider {
position: absolute;
cursor: pointer;
top: 0;
left: 0;
right: 0;
bottom: 0;
background-color: #ccc;
-webkit-transition: .4s;
transition: .4s;
}
.slider:before {
position: absolute;
content: "";
height: 26px;
width: 26px;
left: 4px;
bottom: 4px;
background-color: white;
-webkit-transition: .4s;
transition: .4s;
}
input:checked + .slider {
background-color: #2196F3;
}
input:focus + .slider {
box-shadow: 0 0 1px #2196F3;
}
input:checked + .slider:before {
-webkit-transform: translateX(26px);
-ms-transform: translateX(26px);
transform: translateX(26px);
}
/* Rounded sliders */
.slider.round {
border-radius: 34px;
}
.slider.round:before {
border-radius: 50%;
}
/* Normal Mode*/
body {
background-color: white;
color: black;
}
/* Dark mode */
body.dark-theme {
background-color: black;
color: white;
}

<!-- language: lang-html -->

<label for="change">Dark mode</label>
<label class="switch">
<input type="checkbox" id="change">
<span class="slider round"></span>
</label>

<!-- end snippet -->

input:checked + .slider:before {
-webkit-transform: translateX(26px);
-ms-transform: translateX(26px);
transform: translateX(26px);
}
/* Rounded sliders */
.slider.round {
border-radius: 34px;
}
.slider.round:before {
border-radius: 50%;
}

<!-- language: lang-html -->

<label class="switch">
<input type="checkbox">
<span class="slider round"></span>
</label>

<!-- end snippet -->