Use my recent code as an example:
the css file is in: node_modules/@geoman-io/leaflet-geoman-free/dist/leaflet-geoman.css
node_modules/@geoman-io/leaflet-geoman-free/dist/leaflet-geoman.css
I use: import '@geoman-io/leaflet-geoman-free/dist/leaflet-geoman.css'
import '@geoman-io/leaflet-geoman-free/dist/leaflet-geoman.css'