How can I make my socket.id/nodejs server serve CSS files with the correct mime-type?

Question

I'm using socket.io and https://socket.io/get-started/chat/ to produce a very basic chat server on a remote desktop running Debian. When I access the server's "main" page on port 3000, it fails to load the CSS ("Refused to apply style from '[...].css' because its MIME type ('text/html') is not a supported stylesheet MIME type, and strict MIME checking is enabled."), but it works fine on port 80 when served by Apache.

I'd rather not turn strict mode off, though, since that's more of a workaround than anything.