Basic CSS: Give a Background Color to a div Element

<link href="https://fonts.googleapis.com/css?family=Lobster" rel="stylesheet" type="text/css">
<style>
.red-text {
color: red;
}

h2 {
font-family: Lobster, monospace;
}

p {
font-size: 16px;
font-family: monospace;
}

.thick-green-border {
border-color: green;
border-width: 10px;
border-style: solid;
border-radius: 50%;
}

.smaller-image {
width: 100px;
}
</style>

<h2 class="red-text">CatPhotoApp</h2>
<main>
<p class="red-text">Click here to view more <a href="#">cat photos</a>.</p>

<a href="#"><img class="smaller-image thick-green-border" src="https://bit.ly/fcc-relaxing-cat" alt="A cute orange cat lying on its back."></a>

<div>
<p>Things cats love:</p>
<ul>
<li>cat nip</li>
<li>laser pointers</li>
<li>lasagna</li>
</ul>
<p>Top 3 things cats hate:</p>
<ol>
<li>flea treatment</li>
<li>thunder</li>
<li>other cats</li>
</ol>
</div>

<form action="/submit-cat-photo">
<label><input type="radio" name="indoor-outdoor" checked> Indoor</label>
<label><input type="radio" name="indoor-outdoor"> Outdoor</label><br>
<label><input type="checkbox" name="personality" checked> Loving</label>
<label><input type="checkbox" name="personality"> Lazy</label>
<label><input type="checkbox" name="personality"> Energetic</label><br>
<input type="text" placeholder="cat photo URL" required>
<button type="submit">Submit</button>
</form>
</main>