A simple static file server would look like this:
package main
import (
"net/http"
)
func main() {
muxer := http.NewServeMux()
fileServerCss := http.FileServer(http.Dir("src/css"))
fileServerJs := http.FileServer(http.Dir("src/js"))
fileServerHtml := http.FileServer(http.Dir("content"))
muxer.Handle("/", fileServerHtml)
muxer.Handle("/css", fileServerCss)
muxer.Handle("/js", fileServerJs)
http.ListenAndServe(":8080", muxer)
}