= Create a Theme
:page-aliases: extend-theme.adoc
:description: Create a PDF theme by extending a built-in theme or starting one from scratch.

Asciidoctor PDF allows you to create your own theme, which you define using a YAML data file.
When creating a theme, you have the option to extend a built-in theme and gradually layer on your customizations or to develop a theme from scratch.
This page explains both approaches and the advantages of each.

WARNING: If you do not extend the default theme, we strongly recommend xref:custom-font.adoc[declaring TrueType fonts] in the font catalog and using them in the `base` and `codespan` categories.
Otherwise, Asciidoctor PDF will fall back to the built-in AFM fonts in PDF, which can result in missing functionality and warnings.

== New theme basics

An Asciidoctor PDF theme is defined using a YAML data file that ends with `-theme.yml` (e.g., [.path]_my-theme.yml_).
No keys are actually required.
Each key you specify will customize the styling in some way that differs from the intrinsic defaults.

For example, you can define a theme to set the page size to Letter (which differs from the default, which is A4).

.my-theme.yml
[,yaml]
----
page:
  size: Letter
----

However, a far more powerful approach is to extend another theme, typically a built-in theme.

A theme can declare that it extends--in other words, builds on top of--another theme (or themes) using the `extends` theme key.
The `extends` key must be specified at the top of the theme file.
The theme(s) that your theme extends can be another custom theme or a built-in theme that Asciidoctor PDF provides.

When you extend a theme, that theme is loaded prior to loading the keys in your theme.
If you extend multiple themes, those themes are loaded in the order specified in the `extends` key.
You only need to specify keys to adjust theme settings you want to customize from those that are inherited.

Refer to <<how-extend-works>> for details about what values the `extends` key accepts and how those themes are loaded.

[#extend-default]
== Extend the default theme

*The quickest and simplest way to create a new theme is to extend the default theme.*
Right out of the gate, your theme already has all the functionality provided by the default theme.
You only have to define keys in your theme that you want to customize from the default theme.

Let's create a theme that drastically modifies the base font color.
That way, you can clearly tell when Asciidoctor PDF is using your theme.

Create a new file named [.path]_my-theme.yml_ and populate it with the following code:

.my-theme.yml
[,yaml]
----
extends: default
base:
  font-color: #FF0000
----

Extending the default theme is useful if you want to modify the behavior of the converter or the running content without otherwise changing the style.
For example, here's another theme that customizes the text in the running footer.

[,yaml]
----
extends: default
footer:
  recto:
    right:
      content: A recto page
  verso:
    left:
      content: A verso page
----

Learn more about how to customize the running content on the xref:add-running-content.adoc[] page.

== Extend the base theme

If you're trying to carefully tailor the appearance of the PDF, you might find that the default theme gets in the way too much.
In this case, it might work better to extend the base theme.

The base theme provides rudimentary styling so the visual hierarchy and styling block and inline elements of the content are honored, but does very little to embellish the appearance otherwise.

Here's an example of a theme that extends the base theme:

.my-theme.yml
[,yaml]
----
extends: base
page:
  layout: portrait
  margin: [0.75in, 1in, 0.75in, 1in]
  size: Letter
base:
  font-color: #333333
  font-family: Times-Roman
  font-size: 12
  line-height-length: 17
  line-height: $base-line-height-length / $base-font-size
role:
  removed:
    font-style: italic
    text-decoration: line-through
    text-decoration-color: #FF0000
heading:
  font-color: #262626
  font-size: 17
  font-style: bold
  line-height: 1.2
  margin-bottom: 10
link:
  font-color: #002FA7
list:
  indent: $base-font-size * 1.5
----

Since this theme does not declare any fonts, the built-in AFM fonts in PDF (Helvetica and Courier) will be used.
Even though you did not extend the default theme, you can still configure your theme to use the xref:font-support.adoc#bundled[bundled TrueType fonts] provided by Asciidoctor PDF, as shown in this next theme.

.my-theme.yml
[,yaml]
----
extends: base
font:
  catalog:
    Noto Serif:
      normal: GEM_FONTS_DIR/notoserif-regular-subset.ttf
      bold: GEM_FONTS_DIR/notoserif-bold-subset.ttf
      italic: GEM_FONTS_DIR/notoserif-italic-subset.ttf
      bold_italic: GEM_FONTS_DIR/notoserif-bold_italic-subset.ttf
    M+ 1mn:
      normal: GEM_FONTS_DIR/mplus1mn-regular-subset.ttf
      bold: GEM_FONTS_DIR/mplus1mn-bold-subset.ttf
      italic: GEM_FONTS_DIR/mplus1mn-italic-subset.ttf
      bold_italic: GEM_FONTS_DIR/mplus1mn-bold_italic-subset.ttf
base:
  font-family: Noto Serif
code:
  font-family: M+ 1mn
codespan:
  font-family: $code-font-family
kbd:
  font-family: $code-font-family
button:
  font-family: $base-font-family
----

By layering in the bundled fonts, this extended base theme gives you the most basic starting point without having to worry about providing rudimentary styling.

== Create a theme from scratch

If you want to go even more barebones, you can develop a theme from scratch.
To do so, set the `extends` key to `~` (or omit the key entirely) so Asciidoctor PDF will not load any theme before your own.

.my-theme.yml
[,yaml]
----
extends: ~
#...
----

Although no theme keys are set in this case, Asciidoctor PDF will still resort to using fallback values when a theme setting is required.
Thus, "`from scratch`" really means mostly from scratch.

If you choose not to extend a theme, you should consult the {url-project-repo}/blob/{page-origin-refname}/data/themes/base-theme.yml[base theme^] to discover which keys you'll need to set to support the visual hierarchy and styling of core block and inline elements.
You can also find the location of the [.path]_data/themes_ directory on your local disk by running the following command:

 $ gem contents asciidoctor-pdf --show-install-dir

We strongly recommend extending either the default or base theme at first, and only starting from scratch if you find that approach isn't working out.
That's because developing a theme from scratch takes a lot of effort.

[#how-extend-works]
== How the extends key works

The `extends` key accepts either a single value or an array of values.
Each value is interpreted as either a theme name or filename.
If you don't want to extend any theme (not even the base theme), omit the `extends` key or assign the value `~` to the `extends` key.

If the value matches the name of a xref:index.adoc#built-in-themes[built-in theme] (e.g., `default`), that theme is used.
If the value is an absolute path, that theme file is used.
If the value begins with `./`, the value is resolved to a theme file relative to the current theme file.
Otherwise, the value is resolved just like the value of the `pdf-theme` attribute.
In this case, a relative path is resolved starting from the value of the `pdf-themesdir` attribute.

[#load-theme-more-than-once]
If the same theme appears multiple times in the theme hierarchy, it will only be loaded once by default.
You can force the theme to be loaded, even if it has already been loaded, by adding the `!important` keyword offset by a space to the end of the value.

Initially, the theme starts out empty.
Then, the theme file(s) referenced by the `extends` key are loaded in order.
Finally, the keys in the current file are loaded.
Each time a theme is loaded, the flattened keys are overlaid onto the keys from the previous theme.

Once a key in a theme is processed, all variables are expanded.
That means that if you change the value of a variable after the theme is loaded, the earlier reference to that variable will not be updated.
Instead, you need to redefine the key in order to use the new value of the variable.
