Import css in node_modules to svelte

Question

I had some trouble following Marvelous Walrus’s answer, so here’s a more
explicit version of the same steps.

Rollup is the bundler used by svelte. (You might know webpacker as a bundler).

Step 1: Install the rollup css plugin:

npm install -D rollup-plugin-css-only

Step 2: edit rollup.config.js

you need to import the plugin you just installed at the beginning of rollup.config.js:

import css from "rollup-plugin-css-only";

and farther down in the file you need to find the array of plugins, and add a new on called css. For example I inserted it before the svelte plugin, which looks like this:

  plugins: [
    css({ output: "public/build/extra.css" }),
    svelte({ ...

When rollup does it’s thing, it will read all the css-files imported by your components,
combine them into one, and write them to public/build/extra.css

Step 3: edit public/index.html

Add the link to the new css file. For me this looked like this:

<link rel="icon" type="image/png" href="https://stackoverflow.com/favicon.png" />
<link rel="stylesheet" href="http://stackoverflow.com/build/extra.css" />
<link rel="stylesheet" href="http://stackoverflow.com/global.css" />
<link rel="stylesheet" href="http://stackoverflow.com/build/bundle.css" />

Step 4: profit!

Now you are all set to use css files from your installed npm packages.
for example I added bootstrap by

npm install bootstrap

then finding the css files I want to use (they were in /node_modules/bootstrap/dist/css/)
and importing them in the beginning of App.svelte:

<script>
  // build/extra.css is fed from css files imported here
  import "../node_modules/bootstrap/dist/css/bootstrap.min.css";
  import "../node_modules/bootstrap/dist/css/bootstrap-grid.min.css";

Open Questions

Rollup does not seem to handle the sourcemaps that are mentioned
in bootstrap.min.css and bootstrap-grid.min.css. So when I open the
developer tools I get a warning about not being able to load the sourcemaps

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