How do I configure LESS with Aurelia ESnext Easy-Webpack?

Question

I'm trying to configure Less CSS support in the Aurelia ESnext-Webpack Skeleton starter kit, and having pulled in a Less-based framework, I'm seeing the following warning when I run npm start:

WARNING in ./~/css-loader!./~/spectre.css/spectre.less
Module build failed: Unknown word (1:1)
> 1 | // Copyright 2016 Spectre.css
    | ^
  2 | // Licensed under MIT Open Source
  3 |
  4 | // Core variables

 @ ./src ^\.\/.*$
 @ ./~/aurelia-loader-webpack/dist/commonjs/aurelia-loader-webpack.js
 @ multi aurelia

Here's the configuration I'm using:

/* ./webpack.config.babel.js */
...
import html from '@easy-webpack/config-html'
import less from '@easy-webpack/config-less'
import css from '@easy-webpack/config-css'
...
let config = generateConfig(
  ...
  html(),
  less({ filename: 'styles.css', allChunks: true, sourceMap: false }),
  css({ filename: 'styles.css', allChunks: true, sourceMap: false }),
  ...
)
...

However, despite the warning it appears to be working - styles.css contains all of the less assets when inspected in the browser.

Have I configured easy-webpack/config-less correctly?

If so, how do stop the warning message from showing?