For development I recommend to install packages needed by a buildhost, documented here

sudo -H ./utils/searxng.sh install buildhost

I did install NPM... then run $make styles, but got an error...

You don't need to install NPM on your OS. Beside from the buildhost packages from above, the SearXNG developer environment is build up in the ./local and ./nvm folder (independent from the OS packages).

To get a node environment (in ./nvm) use:

make node.env

To build & commit the /static files (including less styles) use:

make static.build.commit

But first read Development Quickstart carefully 👍