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HTML

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="utf-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    
    <meta name="description" content="">
    <meta name="author" content="">
    <link rel="icon" href="../../favicon.ico">

    <title>Altering height of Bootstrap panel footer</title>

    <!-- Bootstrap core CSS -->
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css">

<style>
body {
  padding-top: 50px;
}
.spacer {
  margin-top: 2%;
  margin-bottom: 2%;
}
.panel-heading{
    height:50px;
} 
.panel-body{
    min-height:150px; 
    background-color: gray;
}     
.panel-footer{
    height:100px; 
    background-color: chartreuse;
}
    
</style>

</head>

<body>

    <nav class="navbar navbar-inverse navbar-fixed-top ">
      <div class="container">
        <div class="navbar-header">
          <button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#navbar" aria-expanded="false" aria-controls="navbar">
            <span class="sr-only">Toggle navigation</span>
            <span class="icon-bar"></span>
            <span class="icon-bar"></span>
            <span class="icon-bar"></span>
          </button>
          <a class="navbar-brand " href="#">Project name</a>
        </div>
        <div id="navbar" class="collapse navbar-collapse">
          <ul class="nav navbar-nav navbar-right">
            <li class="active"><a href="#">Home</a></li>
            <li><a href="#about">About</a></li>
            <li><a href="#contact">Contact</a></li>
          </ul>
        </div><!--/.nav-collapse -->
      </div>
    </nav>

<div class="container col-lg-12 spacer"></div>
    
<div class="container">
    <div class="row">
        <div class="panel panel-primary text-center">
            <div class="panel-heading">
                <h4>Panel Header</h4>
            </div>
           ...