How to avoid serving template files by Go

I'm writting small website on Go, and i found some problems, that i dont know how to solve. So...
The basic idea is to have one single folder for themes, called /themes/ where we will put all our themes, f.e. classic, modern, etc.
The directory tree will looks like:

/themes/
    classic/
        index.html
        header.html
        footer.html
        css/
            style.css
        js/
            lib.js
    modern/
        index.html
        header.html
        footer.html
        css/
            style.css
        js/
            lib.js

So, my http handlers:

func main() {
	reloadConfig()

	http.HandleFunc("/", homeHandler)

	http.HandleFunc("/reloadConfigHandler/", reloadConfigHandler)

	// TODO: Theme loads html files also
	http.Handle("/static/", http.StripPrefix("/static/", http.FileServer(http.Dir("themes/"+config.Theme+"/"))))
	http.ListenAndServe(":80", nil)
}

The Problem

The problem is that my templates files can be opened from the outside, if i open path http://localhost/static/index.html, so i need solution to:

1. Deny /static/, show 404.
2. Deny /static/*.html, show 404.
3. Allow /static/{folder_name}/{file_name} so in future i can add img folder or fonts folder, and content inside of them will be served by server to client.

Thanks in advice.

Easy way is to implement your own http.FileSystem:

type fileSystem struct {
	http.FileSystem
}

func (fs fileSystem) Open(name string) (http.File, error) {
	f, err := fs.FileSystem.Open(name)
	if err != nil {
		return nil, err
	}

	stat, err := f.Stat()
	if err != nil {
		return nil, err
	}

	// This denies access to the directory listing
	if stat.IsDir() {
		return nil, os.ErrNotExist
	}

	// This denies access to anything but <prefix>/css/...
	if !strings.HasPrefix(name, "/css/") {
		return nil, os.ErrNotExist
	}

	return f, nil
}

Now you can use it in your main like so:

fs := http.FileServer(fileSystem{http.Dir("themes/"+config.Theme+"/")})    
http.Handle("/static/", http.StripPrefix("/static/", fs))