What am I doing wrong on this dark mode button?

I started coding today and I'm trying to make a simple static dark mode button, but the background of the button doesn't change its color, I already fixed one thing on the line 36 on CSS it was --bg: var(--green); but the screen would be black when I clicked (https://i.stack.imgur.com/CYUB4.png):

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>site</title>
    <link rel="stylesheet" href="main.css">
</head>
<body class="light-theme">
    <h1>Task List</h1>
    <p id="msg">Current tasks:</p>
    <ul>
        <li class="list">Add visual styles</li>
        <li class="list">Add light and dark themes</li>
        <li> Enable switching the theme</li>
    </ul>
    <div>
        <button class="btn">Dark</button>
    </div>
    <script src="app.js" defer></script>
    <noscript>You need to enable JavaScript to view the full site.</noscript>
</body>
</html>

body {
    font-family: monospace;
}
ul {
    font-family: Arial, Helvetica, sans-serif;
}

li {
    list-style: circle;
}

.list {
    list-style: square;
}

.light-theme {
    color: black;
    background: #00FF00;
    
}

:root {
    --green: #00FF00;
    --white: #ffffff;
    --black: #000000;
}

.light-theme {
    --bg: var(--green);
    --fontColor: var(--black);
    --btng: var(--black)
    --btnFontColor: var(--white)
}

.dark-theme {
    background: var(--black);
    --fontColor: var(--green);
    --btnBg: var(--white)
    --btnFontColor: var(--black)
}

* {
    color: var(--fontColor);
    font-family: Arial, Helvetica, sans-serif;
}

body  {
    background: var(--bg);
}

.btn {
    color: var(--btnFontColor);
    background-color: var(--btnBg);
}

.btn {
    position: absolute;
    top: 20px;
    left: 250px;
    height: 50px;
    width: 50px;
    border-radius: 50%;
    border: none;
    color: var(--btnFontColor);
    background-color: var(--btnBg);
}


.btn:focus { outline: none; }

 'use scrict'

const switcher = document.querySelector('.btn');

switcher.addEventListener('click', function() {
    document.body.classList.toggle('dark-theme')

    var className = document.body.className;
    if(className == "light-theme") {
        this.textContent = "Dark";
    }   
    else {
        this.textContent = "Light"
    }

    console.log('current class name: ' + className);
});
enter image description here

How it was supposed to be: (https://i.stack.imgur.com/SGUMf.png)

Solution

You forgot to add the ";" at the end of the CSS properties in .light-theme and .dark-theme, and in .light-theme you had --btng rather than --btnBg.

.light-theme {
    --bg: var(--green);
    --fontColor: var(--black);
    --btnBg: var(--black);
    --btnFontColor: var(--white);
}

.dark-theme {
background: var(--black);
    --fontColor: var(--green);
    --btnBg: var(--white);
    --btnFontColor: var(--black);
}