<require from="bootstrap/css/bootstrap.css"></require>

"overrides": {
   "npm:jquery@^3.0.0": {
     "format": "amd"
   }
}

"dependencies": [
"jquery",
 {
    "name": "bootstrap",
    "path": "../node_modules/bootstrap/dist",
    "main": "js/bootstrap.min",
    "deps": ["jquery"],
    "exports": "$",
    "resources": [
    "css/bootstrap.css"
  ]
}
]

"bundles": [
  {
    "name": "app-bundle.js",
    "source": [
      "[**/*.js]",
      "**/*.{css,html}"
    ],
    "dependencies": [
      "jquery",
      {
        "name": "bootstrap",
        "path": "../node_modules/bootstrap/dist",
        "main": "js/bootstrap.min",
        "deps": ["jquery"],
        "exports": "$",
        "resources": [
          "css/bootstrap.css"
        ]
      }
    ]
  },

"jquery",
{
  "name": "bootstrap",
  "path": "../node_modules/bootstrap/dist",
  "main": "js/bootstrap.min",
  "deps": [
    "jquery"
  ],
  "resources": [
    "css/bootstrap.css"
  ],
  "exports": "$"
}

"copyFiles": {
  "node_modules/bootstrap/dist/fonts/glyphicons-halflings-regular.woff2": "bootstrap/fonts",
  "node_modules/bootstrap/dist/fonts/glyphicons-halflings-regular.woff": "bootstrap/fonts",
  "node_modules/bootstrap/dist/fonts/glyphicons-halflings-regular.ttf": "bootstrap/fonts"
}

<require from="bootstrap/css/bootstrap.css"></require>

aurelia.use
    .standardConfiguration()
      .feature('resources')
      .globalResources('bootstrap/css/bootstrap.css');

<require from="bootstrap/css/bootstrap.css"></require>

<require from="bootstrap/dist/css/bootstrap.css"></require>