main.go

package main

import (
	"github.com/gorilla/mux"
	"html/template"
	"log"
	"net/http"
)

func main () {
	route := mux.NewRouter()

	http.FileServer(http.Dir("/public"))

	route.HandleFunc("/", func(writer http.ResponseWriter, request *http.Request) {
		view, err := template.ParseFiles("views/errors/404.html")

		if err != nil {
			log.Fatal(err)
		}

		_ = view.Execute(writer, nil)
	})

	err := http.ListenAndServe(":9000", route)
	if err != nil {
		log.Fatal(err)
	}
}

package main

import (
	"log"
	"net/http"

	"github.com/gorilla/mux"
)

func main() {
	route := mux.NewRouter()

	route.PathPrefix("/assets/").Handler(http.StripPrefix("/assets/", http.FileServer(http.Dir("./public"))))

	err := http.ListenAndServe(":9000", route)
	if err != nil {
		log.Fatal(err)
	}
}

This works. You need to have a public directory directly at the same level in the filesystem as the binary you are running. So if a file is in ./public/css/index.css, it will be accessed by <a href="/assets/css/index.css">

You can change the paths obviously