How to change sytle properties of an element in JavaScript?

I am making a small online app that will require the user to input some options and the site will change it’s background and similar features based on their input.
However, I have not been able to change any of the document style properties through JavaScript code.

Am I missing something? 

let bgType = prompt("Which type of background do you want? Color or image: ");

if (bgType == "color" || bgType == "Color") {
    let colBg = prompt("Enter the color you want");
    document.getElementById("background").style.backgroundColor = colBg; // I've tried putting default colors here for testing, but those didn't work either
} else if (bgType == "image" || bgType == "Image") {
    let imgBg = prompt("Enter the number of the image you want (1-3): ");
    document.getElementById("background").style.backgroundColor = ""; // Unfinished
}

In case it’s needed here is also the HTML and CSS code
HTML:

<!DOCTYPE html>
<html>
    <head>
        <meta charset='utf-8'>
        <meta http-equiv='X-UA-Compatible' content='IE=edge'>
        <title>Greeting card generator</title>
        <meta name='viewport' content='width=device-width, initial-scale=1'>
        <link rel='stylesheet' type='text/css' media='screen' href='gfx.css'>
    </head>
    <body>
        <script src='script.js'></script>
        <div id="background" class="content">
            <!-- <img src="Temp1.jpg" alt="Greeting card image"> -->
            <h1 id="Title" class="text">
                Sample title text
            </h1>
            <h2 id="Name" class="text">
                Sample name text
            </h2>
            <h3 id="Message" class="text">
                Sample message text
            </h3>
        </div>
    </body>
</html>

CSS:

html, body {
    height: 100%;
    margin: 0;
}

#background {
    width: 100%;
    min-height: 100%;
    background-image: url();
    background-color: white;
    background-size: cover;
    background-position: center;
    background-repeat: no-repeat;
}

.text {
    font-family: verdana;
    text-align: center;
}

#Title {
    padding-top: 20%;
}