Changing the margins on Bootstrap 3 for container

You can simply override the CSS. However, you should avoid modifying the Bootstrap files directly, as that limits your ability to update the library. Place your own, custom CSS after Bootstrap, and modify it however you choose.

Further, try using SASS or LESS and creating a variable for your margins/padding. Then you can reuse the variable for various breakpoints or custom containers, and have a single point to edit the margins/padding later.

Another good idea is to modify your containers with a custom class, so that the original styles are preserved. For example:

<style type="text/css">
    .container.custom-container {
      padding: 0 50px;
    }
</style>

<div class="container">
  Here's a normal container
</div>

<div class="custom-container container">
  Here's a custom container
</div>

In bootstrap 4, container-fluid gives a full-width container