How to specify folder with css files in webpack?

Question

css

js are stored in /_source/frontend/app
css, in /_source/frontend/css
for js I specify the location of the start file in entry,
but what about css?
if you specify test: /\.css/ in the loaders, then the files will be searched in /_source/frontend/app, which I don’t need
to collect everything using gulp before, and everything is much easier there.