Theming Strategies

The storybook gives you different possibilities to apply a theme to your components. These possibilities are named strategies.

The following strategies are available:

1. sandbox class: set your theme as a CSS class, on the sandbox container, with a custom prefix
2. root class (root_class): set your theme as a CSS class on the root html element, with a custom prefix.
3. data attribute (data_attribute): set your theme on the sandbox container as a data-* attribute.
4. assign: pass the theme as an assign to your components, with a custom key.
5. function: call a custom module/function along with the current theme.

Here is how you can use these strategies. In your storybook.ex:

use PhoenixStorybook,
  themes_strategies: [
    sandbox_class: "prefix", # will set a class prefixed by `prefix-` on the sandbox container
    root_class: "theme", # will set `theme-<theme name>` on the root html element
    data_attribute: "name", # will set data-name="theme" on the sandbox container
    assign: :theme,
    function: {MyApp.ThemeHelper, :register_theme}
  ]

If the themes_strategies key is undefined, the default sandbox_class: "theme" strategy is applied.

CSS theming

By default, the storybook is applying a theme-* CSS class to your components/page containers and you should do as well to your application HTML body element.

It will allow you to style raw HTML elements

body.theme-colorful {
  font-family: // ...
}

.theme-colorful h1 {
  font-family: // ...
  font-size: // ...
}

Using a Registry

This chapter explain how you can leverage on a Registry with the function theming strategy.

An effective way to store the current theme setting so that it can be available to all your components, but still have different values for different (concurrent) users is to associate it to the current LiveView pid.

Registry is a native Elixir module that handles decentralized storage, linked to specific processes. We will leverage on this to associate a theme to the current LiveView pid.

First start a Registry from your Application module.

defmodule PhenixStorybook.Application do
  def start(_type, _args) do
    children = [
      {Registry, keys: :duplicate, name: ThemeRegistry}
    ]
  end
end

Then create a LiveView Hook that will fetch the theme from wherever it is relevant for your application: database, user session, URL params... and store it in the Registry (it's working because the Hook is running under the same pid than the LiveView).

defmodule ThemeHook do
  def on_mount(:default, params, _session, socket) do
    theme = current_user_theme(socket, params)
    Registry.register(ThemeRegistry, :theme, theme)
    {:cont, socket}
  end
end

Mount the hook in your router.

defmodule Router do
  live_session :default, on_mount: [ThemeHook] do
    scope "/" do
      # ...
    end
  end
end

Write a helper module, to be used from your components to fetch the current theme from the Registry and merge it in the component's assigns.

defmodule ThemeHelpers do
  def set_theme(assigns) do
    pid_and_themes = Registry.lookup(ThemeRegistry, :theme)

    case find_by_pid(pid_and_themes, self()) do
      {_pid, theme} -> Map.put_new(assigns, :theme, theme)
      _ -> raise("theme not found in registry")
    end
  end

  defp find_by_pid(pid_and_themes, current_pid) do
    Enum.find(pid_and_themes, fn {pid, _} -> pid == current_pid end)
  end
end

Theming the storybook UI

Everything above themes your components inside the sandbox. The storybook chrome itself (sidebar, header, playground, docs…) has its own theme, built from a small set of shadcn-style semantic tokens exposed as CSS custom properties (--psb-color-*, plus a --psb-radius scale) and psb:-prefixed Tailwind utilities. You can recolor and reskin it to match your brand.

The theme file is built by Tailwind and served like your other assets, then imported by the storybook unlayered, after its own stylesheet. Because it is unlayered, your rules win over both the shipped token defaults and the psb:* utilities baked into the markup — in light and dark mode. This lets you:

• recolor by overriding tokens (--psb-color-*, --psb-radius), and
• reskin by overriding structural rules (borders, spacing, shadows…).

Two stylesheets, two jobs

mix phx.gen.storybook scaffolds two separate CSS files, and it is easy to mix them up. They paint different surfaces and never overlap:

The rule of thumb: if you are styling something a story renders, it goes in storybook.css. If you are styling the storybook interface around those stories, it goes in storybook_theme.css.

storybook.css carries whatever your components need — PhoenixStorybook loads it instead of your app.css, so any @plugin, @custom-variant, font, or @source you rely on must be mirrored there:

/* assets/css/storybook.css — themes your components */
@import "tailwindcss" source(none);
@source "../../storybook";

.my-app-sandbox h1 {
  font-family: "Inter", sans-serif;
}

storybook_theme.css only overrides the chrome's tokens and rules — no @source, no component utilities:

/* assets/css/storybook_theme.css — themes the storybook UI */
:root {
  --psb-color-primary: #0ea5e9;
}

The rest of this section is about storybook_theme.css. For storybook.css and the sandbox, see the sandboxing guide.

The theme file

mix phx.gen.storybook scaffolds an annotated assets/css/storybook_theme.css for you (also available at priv/templates/phx.gen.storybook/storybook_theme.css). Keep the tokens you want to change, and delete the rest:

/* assets/css/storybook_theme.css */
:root {
  --psb-color-primary: #0ea5e9;
  --psb-radius: 0.375rem;
}

.psb\:dark {
  --psb-color-primary: #38bdf8;
}

Notes:

• Override the prefixed names (--psb-color-*), not the unprefixed --color-* names.
• Values can be raw (#0ea5e9, oklch(...)) or reference the bundled palette (var(--psb-color-sky-500)).
• The dark-mode block is keyed on .psb\:dark (note the escaped colon).

Reskinning with plain rules or @apply

Since the file is unlayered, a plain rule overrides the utilities in the markup. For example, to drop the border around the playground component preview:

:has(> [id$="-playground-preview"]) {
  border-width: 0;
}

To reuse the storybook's own psb:* utilities via @apply, reference the storybook's Tailwind setup first — this emits no CSS, it only makes the utilities and --psb-color-* theme available to the compiler:

@reference "../../deps/phoenix_storybook/priv/phoenix_storybook.css";

:has(> [id$="-playground-preview"]) {
  @apply psb:border-0 psb:shadow-none;
}

Wiring it up

The theme file goes through your asset build, exactly like css_path. The generator sets theme_path in your backend and prints the build steps to add (a storybook_theme Tailwind profile, a dev watcher, and assets.build / assets.deploy alias entries):

use PhoenixStorybook,
  otp_app: :my_app,
  # remote path, served from priv/static — not a local file-system path
  theme_path: "/assets/css/storybook_theme.css"

The built file's fingerprint is tracked as an external resource, so rebuilding it busts the cache; if theme_path points at a file that has not been built yet, the storybook logs a warning and falls back to its shipped defaults.