Document how to customize appearance of DocC docs (#359)

Resolves: rdar://96976426
Resolves: rdar://96977013

Author: mportiz08

Sources/DocCDocumentation/DocCDocumentation.docc/Assets/theme-screenshot-01.png

@@ -29,6 +29,7 @@ DocC syntax — called _documentation markup_ — is a custom variant of Markdow
 ### Distribution
 
 - <doc:distributing-documentation-to-other-developers>
+- <doc:customizing-the-appearance-of-your-documentation-pages>
 
 ### Documentation Types
 

Sources/DocCDocumentation/DocCDocumentation.docc/Assets/theme-screenshot-02.png

@@ -0,0 +1,243 @@
+# Customizing the Appearance of Your Documentation Pages
+
+Customize the look and feel of your documentation webpages.
+
+## Overview
+
+By default, rendered documentation webpages produced by DocC come with a default
+visual styling. If you wish, you may make adjustments to this styling by adding
+an optional `theme-settings.json` file to the root of your documentation catalog
+with some configuration. This file is used to customize various things like
+colors and fonts. You can even make changes to the way that specific elements
+appear, like buttons, code listings, and asides. Additionally, some metadata for
+the website can be configured in this file, and it can also be used to opt in or
+opt out of certain default rendering behavior.
+
+If you're curious about the full capabilities of the `theme-settings.json` file,
+you can check out the latest Open API specification for it [here][1], which
+documents all valid settings that could be present.
+
+Let's walk through some basic examples of customizing the appearance of
+documentation now.
+
+### Colors
+
+DocC utilizes [CSS custom properties][2] (CSS variables) to define the various
+colors that are used throughout the HTML elements of rendered documentation. Any
+key/value in the `theme.color` object of `theme-settings.json` will either
+update an existing color property used in the renderer or add a new one that
+could be referenced by existing ones.
+
+**Example**
+
+As a simple example, the following configuration would update the background
+fill color for the "documentation intro" element to `"rebeccapurple"` by
+changing the value of the CSS property to `--color-documentation-intro-fill:
+"rebeccapurple"`.
+
+```json
+{
+  "theme": {
+    "color": {
+      "documentation-intro-fill": "rebeccapurple"
+    }
+  }
+}
+```
+
+![A screenshot of the documentation intro element with a purple background color](theme-screenshot-01.png)
+
+**Example**
+
+Note that any valid CSS color value could be used. Here is a more advanced
+example which demonstrates how specialized dark and light versions of the main
+background fill and nav colors could be defined as varying lightness percentages
+of a green hue.
+
+```json
+{
+  "theme": {
+    "color": {
+      "hue-green": "120",
+      "fill": {
+        "dark": "hsl(var(--color-hue-green), 100%, 10%)",
+        "light": "hsl(var(--color-hue-green), 100%, 90%)"
+      },
+      "fill-secondary": {
+        "dark": "hsl(var(--color-hue-green), 100%, 20%)",
+        "light": "hsl(var(--color-hue-green), 100%, 80%)"
+      },
+      "nav-solid-background": "var(--color-fill)"
+    }
+  }
+}
+```
+
+![A screenshot of the green page background in dark mode](theme-screenshot-02.png)
+
+![A screenshot of the green page background in light mode](theme-screenshot-03.png)
+
+As a general rule, the default color properties provided by DocC assumes a
+naming convention where "fill" colors are used for backgrounds and "figure"
+colors are used for foreground colors, like text.
+
+> Tip:
+> For a more complete example of a fully customized documentation website, you
+> can check out [this fork][3] of the DocC documentation that is using this
+> [theme-settings.json file][4].
+
+### Fonts
+
+By default, there are two main fonts used by DocC: one for monospaced text in
+code voice and another for all other text content. With `theme-settings.json`,
+you can customize the CSS `font-family` lists for these however you choose.
+
+**Example**
+
+```json
+{
+  "theme": {
+    "typography": {
+      "html-font": "\"Comic Sans MS\", \"Comic Sans\", cursive",
+      "html-font-mono": "\"Courier New\", Courier, monospace"
+    }
+  }
+}
+```
+
+![A screenshot of a documentation page with custom fonts](theme-screenshot-04)
+
+### Borders
+
+Another aspect of the renderer that you may wish to modify is the way that it
+applies border to certain elements. With `theme-settings.json`, you can change
+the default border radius for all elements and also individually update the
+border styles for asides, buttons, code listings, and tutorial steps.
+
+**Example**
+
+```json
+{
+  "theme": {
+    "aside": {
+      "border-radius": "6px",
+      "border-style": "double",
+      "border-width": "3px"
+    },
+    "border-radius": "0",
+    "button": {
+      "border-radius": "8px",
+      "border-style": "solid",
+      "border-width": "2px"
+    },
+    "code": {
+      "border-radius": "2px",
+      "border-style": "dashed",
+      "border-width": "2px"
+    },
+    "tutorial-step": {
+      "border-radius": "12px"
+    }
+  }
+}
+```
+
+![A screenshot of a tutorial page with custom button
+borders](theme-screenshot-05.png)
+
+![A screenshot of tutorial steps with custom borders](theme-screenshot-06.png)
+
+![A screenshot of a documentation page with custom code listing and aside border
+styles](theme-screenshot-07.png)
+
+### Icons
+
+It is also possible to use `theme-settings.json` to specify custom icons to use
+in place of the default ones provided by DocC. There are a few basic steps to
+follow in order to override an icon:
+
+1. Find the identifier that maps to the icon you want to override.
+2. Add this value as an `id` attribute to the new SVG icon.
+3. Associate a relative or absolute URL to the new SVG icon with the identifier
+   in `theme-settings.json`.
+
+> Note:
+> You can find all the possible identifier values for every valid icon kind in
+> the Open API [specification][1] for `theme-settings.json`.
+
+**Example**
+
+As a simple example, here is how you could update the icon used for articles in
+the sidebar to be a simple box:
+
+1. Find the identifier for article icons: `"article"`
+2. Add the `id="article"` attribute to the new SVG icon: `simple-box.svg`
+    ```svg
+    <svg viewBox="0 0 14 14" xmlns="http://www.w3.org/2000/svg" id="article">
+      <rect x="2" y="2" width="10" height="10" stroke="currentColor" fill="none"></rect>
+    </svg>
+    ```
+3. Configure `theme-settings.json` to map the URL for the new article icon
+    ```json
+    {
+      "theme": {
+        "icons": {
+          "article": "/images/simple-box.svg"
+        }
+      }
+    }
+    ```
+
+![A screenshot of the sidebar using a custom icon for
+articles](theme-screenshot-08.png)
+
+### Metadata
+
+You can specify some text that will be used in the HTML `<title>` element for
+all webpages in `theme-settings.json` in place of the default "Documentation"
+text.
+
+**Example**
+
+```json
+{
+  "meta": {
+    "title": "Custom Title",
+  }
+}
+```
+
+With that configuration, the title of this page would change from "Customizing
+Your Documentation Appearance | Documentation" to "Customizing Your Documentation
+Appearance | **Custom Title**".
+
+### Feature Flags
+
+From time to time, there may be instances of various experimental UI features
+that can be enabled or disabled using `theme-settings.json`.
+
+**Example**
+
+As of the time of writing this, there is only one available "quick navigation"
+feature that is disabled by default and can be enabled with the following
+configuration. Note that this flag may be dropped in the future and new ones may
+be added as necessary for other features.
+
+```json
+{
+  "features": {
+    "docs": {
+      "quickNavigation": {
+        "enable": true
+      }
+    }
+  }
+}
+```
+
+[1]: https://github.com/apple/swift-docc/blob/main/Sources/SwiftDocC/SwiftDocC.docc/Resources/ThemeSettings.spec.json
+[2]: https://drafts.csswg.org/css-variables/
+[3]: https://mportiz08.github.io/swift-docc/documentation/docc
+[4]: https://mportiz08.github.io/swift-docc/theme-settings.json
+
+<!-- Copyright (c) 2022 Apple Inc and the Swift Project authors. All Rights Reserved. -->