ActiveAdmin: How to introduce Slicknav menu

active_admin.css.scss

@import "active_admin/mixins";
@import "active_admin/base";
@import "responsive_active_admin";
@import "slicknav";

.slicknav_menu {
    display: none;

    @include responsive {
        display: block;
    }

    .slicknav_btn {
        background-color: #404040;  // the default color looks too dark
    }

    .slicknav_nav {
        h1 {
            display: none;
        }
        ul + ul {
            border-top: 1px solid rgba(0, 0, 0, 0.2);
        }

        li.current a {
            border-radius: 10px;
            background-color: #7B8389;
            color: #fff;
        }
    }
}

#header {
    @include responsive {
        display: none;
    }
}

active_admin.js.coffee

#= require active_admin/base
#= require slicknav

$ ->
  $('#header').slicknav(label: $('#site_title').text())

Gemfile

source 'http://rails-assets.org' do
  gem 'rails-assets-slicknav'
end