Can't edit theme defaults

I tried to add this file to the project directory to create some custom styles as described by the theme documentation, but it doesn't seem to actually override the main css file. Also, when I rebuild the project with Jekyll serve it deletes the file. I added the following lines to the config file as well, but they didn't change anything:

sass: sass_dir: _sass

Scss file:

---
---
$primary: #de0a0a;
//$currentColor: red;
.button.is-primary {
    background-color: #009fdf;
    border-color: transparent;
    color: #fff;
  }
  .button.is-primary:hover, .button.is-primary.is-hovered {
    background-color: #0d0b6e;
    border-color: transparent;
    color: #fff;
  }

  .navbar.is-info .navbar-brand .navbar-link.is-active {
    background-color: #009fdf;
    color: #fff;
  }

  .navbar.is-primary {
    background-color: black;
    color: #fff;
  }

  .navbar.is-primary .navbar-start > a.navbar-item:focus, .navbar.is-primary .navbar-start > a.navbar-item:hover, .navbar.is-primary .navbar-start > a.navbar-item.is-active,
  .navbar.is-primary .navbar-start .navbar-link:focus,
  .navbar.is-primary .navbar-start .navbar-link:hover,
  .navbar.is-primary .navbar-start .navbar-link.is-active,
  .navbar.is-primary .navbar-end > a.navbar-item:focus,
  .navbar.is-primary .navbar-end > a.navbar-item:hover,
  .navbar.is-primary .navbar-end > a.navbar-item.is-active,
  .navbar.is-primary .navbar-end .navbar-link:focus,
  .navbar.is-primary .navbar-end .navbar-link:hover,
  .navbar.is-primary .navbar-end .navbar-link.is-active {
    background-color: #009fdf;
    color: #fff;
  }

  .navbar.is-primary .navbar-start > a.navbar-item:focus, .navbar.is-primary .navbar-start > a.navbar-item:hover, .navbar.is-primary .navbar-start > a.navbar-item.is-active,
  .navbar.is-primary .navbar-start .navbar-link:focus,
  .navbar.is-primary .navbar-start .navbar-link:hover,
  .navbar.is-primary .navbar-start .navbar-link.is-active,
  .navbar.is-primary .navbar-end > a.navbar-item:focus,
  .navbar.is-primary .navbar-end > a.navbar-item:hover,
  .navbar.is-primary .navbar-end > a.navbar-item.is-active,
  .navbar.is-primary .navbar-end .navbar-link:focus,
  .navbar.is-primary .navbar-end .navbar-link:hover,
  .navbar.is-primary .navbar-end .navbar-link.is-active {
    background-color: #009fdf;
    color: #fff;
  }
  .navbar.is-primary .navbar-brand > a.navbar-item:focus, .navbar.is-primary .navbar-brand > a.navbar-item:hover, .navbar.is-primary .navbar-brand > a.navbar-item.is-active,
.navbar.is-primary .navbar-brand .navbar-link:focus,
.navbar.is-primary .navbar-brand .navbar-link:hover,
.navbar.is-primary .navbar-brand .navbar-link.is-active {
  background-color: #009fdf;
  color: #fff;
}


.navbar.is-primary .navbar-item.has-dropdown:focus .navbar-link,
.navbar.is-primary .navbar-item.has-dropdown:hover .navbar-link,
.navbar.is-primary .navbar-item.has-dropdown.is-active .navbar-link {
  background-color: #009fdf;
  color: #fff;
}
.navbar.is-primary .navbar-dropdown a.navbar-item.is-active {
  background-color: #009fdf;
  color: #fff;
}
// Import Main CSS file from theme
@import "main";