I would like to change the background color of the bootstrap theme. This background: To change the background of the first image, change the background property of body element in index.php

body {
    width: 100%;
    height: 100%;
    font-family: 'Open Sans', sans-serif;
    background: red; // here
 }

To change the background of the second image,in sb-admin-2.css

.bg-gradient-info {
  background-color: red; //here, you can override this class
}