Utilize !important
to avoid being overridden:
.form-group {
margin-bottom: 0px!important;
}
Alternatively, incorporate bootstrap 4 spacing:https://getbootstrap.com/docs/4.0/utilities/spacing/
<div class="form-group mb-0">
As an illustration:
.space .form-group {
margin-bottom: 0px!important;
}
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src= "https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>
<form action="/action_page.php">
<h1>Without space</h1>
<div class="space">
<div class="form-group">
<label for="email">Email:</label>
<input type="email" class="form-control" id="email" placeholder="Enter email" name="email">
</div>
<div class="form-grou">
<label for="pwd">Password:</label>
<input type="password" class="form-control" id="pwd" placeholder="Enter password" name="pwd">
</div>
</div>
<h1>With space</h1>
<div class="form-group">
<label for="email">Email:</label>
<input type="email" class="form-control" id="email" placeholder= "Enter email" name="email">
</div>
<div class="form-group">
<label for="pwd">Password:</label>
<input type="password" class="form-control" id="pwd" placeholder="Enter password" name="pwd">
</div>
</form>