Webpack process only picks up some of the css files in a theme

As filed by @Relequestual as cloudflare/doca#40

> When creating a theme with multiple css files in the styles folder, not all of them are always pick up and included.
>
> Noted at https://github.com/cloudflare/doca/issues/39#issuecomment-278031709
>
> I think I found the cause of this problem!
> I had installed bootstrap, but removed the font files.
> The CSS loader had a problem when it couldn't find the fonts, and died, without saying a word.
> I only noticed this when I restarted the server in development mode and saw it in the console while it was trying to reload modules.
> Added the fonts back, no problem.
>
> I still came across a load order issue, but because they are loaded in alphabetical order (it seems), that's easy to fix.
>
> Added another CSS file. Still have the problem where it's not picked up at...
`require.context('doca-bootstrap-theme/styles', true, /\.css$/gi);`
main.js