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`.
- Run `./node_modules/bower/bin/bower update` to update `bower_components`.
- Copy the SASS sources from `bower_components` to `debian/missing-sources`.

The following components are copied:

Name    | Website                  | WNPP bug | Currently used version
------- | ------------------------ | -------- | ----------------------
bourbon | https://www.bourbon.io/  | #892811  | 4.0.2
neat    | https://neat.bourbon.io/ |          | 1.6.0
wyrm    | https://wyrmsass.org/    |          | 1.0.9

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

Using the packaged versions may be tricky, as wyrm currently requires neat 1.6
which is quite old. Neat 1.8 and newer no longer requires bourbon.
