How to change the style for label, warning block, and submit button?

To change the style of the form, just add or change the css in the form.html.

Set maximum width to form

.jf-form > form{
  margin-bottom: 32px;
  max-width: 480px;
}

Change color for field labels

.jf-form label.control-label{
  color: #3276B1;
}

Change background color of warning block

.jf-form .bg-warning{
    background-color:#f7ed53 ;
}

Change background color of submit button

.jf-form .btn-primary{
  background-image: none;
  background-color: #7ab733;
}

.jf-form .btn-primary:hover{
  background-image: none;
  background-color: #3276B1;
}

The form style changed as below screenshot