The `debian/missing-sources` directory contains the SASS sources needed to
build the theme CSS.

The procedure for updating them is as follows:

- Install the latest versions of node.js and npm.
- Install the JavaScript dependencies using `npm install`.
- Copy `bourbon`, `bourbon-neat` and `wyrm` directories from `node_modules`
  to `debian/missing-sources`.

The following components are copied:

Name         | Website                       | WNPP bug | Currently used version
------------ | ----------------------------- | -------- | ----------------------
bourbon      | https://www.bourbon.io/       | #892811  | 4.3.4
bourbon-neat | https://neat.bourbon.io/      |          | 1.9.1
wyrm         | https://github.com/snide/wyrm |          | 1.0.9

Note that only the SASS files and the corresponding documentation are copied,
not the other files.