Thank you for the info @talss89!
Adding your example led to the @import not being touched in the build process
But all urls() like background-image: url('../images/image.png')
became url('webpack:///mini-css-extract-plugin//app/themes/...)
.
I fixed this by changing
app.setPublicPath('/app/themes/mesta-maestarna/dist/');
to
app.setPublicPath(app.env.get('WP_HOME') + '/app/themes/[theme-name]/dist/');
I hear what you are saying and if I run into any more issues, I will try to persuade the other dev to change his mind about this
Regarding having to process the other CSS-files, I have found Bud config for outputting multiple css files (from scss for ACF Blocks) and is hoping to make that process automatic for all css files in a folder named “partials”.