If you want to customize the appearance of the bootstrap well
class, you'll need to override its CSS rules.
To achieve this, create a separate CSS file and link it externally in your HTML page. For instance:
.well {
background-color: rgba(0,0,0,0.4) !important;
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" rel="stylesheet"/>
<div class="well">
Some content
</div>
Hopefully, this solution will be useful for you.
EDIT: You can also include the custom styles within the <style></style> tags on the same page, making sure to place them after the Bootstrap sources:
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>Developing JazZ Sheet</title>
<script src="https://code.jquery.com/jquery-3.1.1.min.js" integrity="sha256-hVVnYaiADRTO2PzUGmuLJr8BLUSjGIZsDYGmIJLv2b8=" crossorigin="anonymous"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
<style>
.well {
background-color: rgba(0,0,0,0.4);
}
</style>
</head>
<body>
<div class="well">
Some content
</div>
</body>
</html>