Add this to bower.json and remove what you don't need

Bootstrap overrides for Sage 8.5.1 and Bootstrap 4 alpha 6

"overrides": {
    "bootstrap": {
      "main": [
        "./scss/_variables.scss",
        "./scss/_mixins.scss",
        "./scss/_custom.scss",
        "./scss/_normalize.scss",
        "./scss/_print.scss",
        "./scss/_reboot.scss",
        "./scss/_type.scss",
        "./scss/_images.scss",
        "./scss/_code.scss",
        "./scss/_grid.scss",
        "./scss/_tables.scss",
        "./scss/_forms.scss",
        "./scss/_buttons.scss",
        "./scss/_transitions.scss",
        "./scss/_dropdown.scss",
        "./scss/_button-group.scss",
        "./scss/_input-group.scss",
        "./scss/_custom-forms.scss",
        "./scss/_nav.scss",
        "./scss/_navbar.scss",
        "./scss/_card.scss",
        "./scss/_breadcrumb.scss",
        "./scss/_pagination.scss",
        "./scss/_badge.scss",
        "./scss/_jumbotron.scss",
        "./scss/_alert.scss",
        "./scss/_progress.scss",
        "./scss/_media.scss",
        "./scss/_list-group.scss",
        "./scss/_responsive-embed.scss",
        "./scss/_close.scss",
        "./scss/_modal.scss",
        "./scss/_tooltip.scss",
        "./scss/_popover.scss",
        "./scss/_carousel.scss",
        "./scss/_utilities.scss",
        "./js/dist/alert.js",
        "./js/dist/button.js",
        "./js/dist/carousel.js",
        "./js/dist/collapse.js",
        "./js/dist/dropdown.js",
        "./js/dist/modal.js",
        "./js/dist/popover.js",
        "./js/dist/scrollspy.js",
        "./js/dist/tab.js",
        "./js/dist/tooltip.js",
        "./js/dist/util.js"
      ]
    }
  }