am working on my odoo 12 i tried to add a custom css file to my custom module, after updating i got this message 

The style compilation failed, see the error below. Your recent actions may be the cause, please try reverting the changes you made.

Could not get content for /MyAddons/immobilier/static/src/css/bootstrap.css defined in bundle 'web.assets_backend'.

This is what i added on my custom css file

.rounded-pill {
  border-radius: 9999px;
}

/* _print.scss:116 */
.table-bordered-sides {
  th,
  td,
  tr {
    border-left: 1px solid $gray-300 !important;
    border-right: 1px solid $gray-300 !important;
  }
}

/* _tables.scss:48 */
.table-bordered-bordered-all-sides {

  border: $table-border-width solid $table-border-color;

  th,
  td {
    border: $table-border-width solid $table-border-color;
  }

  thead {
    th,
    td {
      border-bottom-width: (2 * $table-border-width);
    }
  }

} 

Any help?