:hover

css/style.css

div {
width: 200px;
height: 200px;
background-color: #FFD400;
}
div:hover {
background-color: #3BA9DE;
}

div {
 width: 200px;
 height: 200px;
 background-color: #FFD400;
}

div:hover {
 background-color: #3BA9DE;
}

index.html

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<link href="css/style.css" rel="stylesheet" />
</head>
<body>
<div></div>
</body>
</html>

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<link href="css/style.css" rel="stylesheet" />
</head>
<body>
<div></div>
</body>
</html>