As a sidenote I do it a little bit differently than in the link.

Often I add a helpers folder in lib/any_web/helpers. I add a bootstrap_helper, with corresponding view, and template folder. I add it globally in any_web.ex, like this…

  def view do
    quote do
      use Phoenix.View,
        root: "lib/any_web/templates",
        namespace: AnyWeb

      # Import convenience functions from controllers
      import Phoenix.Controller, only: [get_flash: 1, get_flash: 2, view_module: 1]

      # Use all HTML functionality (forms, tags, etc)
      use Phoenix.HTML

      # Here I import some custom helpers
      import AnyWeb.Helpers.CheckboxHelper
      import AnyWeb.Helpers.BootstrapHelper
      import AnyWeb.Helpers.WaffleHelper
      import AnyWeb.Helpers.BaseHelper

      import AnyWeb.ErrorHelpers
      import AnyWeb.Gettext
      alias AnyWeb.Router.Helpers, as: Routes
    end
  end

These are 2 simple functions I put in helper. One is using content_tag, one render a template.

defmodule AnyWeb.Helpers.BootstrapHelper do
  @moduledoc """
  This is the documentation for the Bootstrap 4 helpers.
  It includes also fa helper, for Font awesome
  """
  use Phoenix.HTML

...

  def flash(message, mode) do
    class = "alert alert-#{mode} alert-dismissible fade show"
    content_tag(:div, class: class, role: "alert") do
      [
        message,
        content_tag(:button, type: :button, class: :close, "data-dismiss": :alert, "aria-label": "Close") do
          content_tag(:span) do
            raw("×")
          end
        end
      ]
    end
  end

  @doc """
  Dropdown helper
  """
  def dropdown(options \\ []) do
    render("dropdown.html", options)
  end

Then, in any template, I have access to helper functions to simplify building components. Like this…

<%= flash("hello world", "alert") %>

The main difference with the linked example is I import helpers…

If one day I decide to change css framework, I would just replace bootstrap helper with a new one.