The `com.elovirta.pdf` plug-in for DITA-OT extends the built-in PDF2 plug-in by adding support for the `theme` property. The `theme` property takes a path to a theme file and changes the styling of the PDF2 output without needing to extend the transtype with XSLT.

To generate PDF output with a custom theme, pass the path to the theme file to the `dita` command with the `--theme` option:

```shell
dita --project=samples/project-files/pdf.xml \
     --theme=/path/to/custom-theme-file.yaml
```

The following topics provide details on the theme file formats and supported configuration options.

## Theme file

A theme file can be written in either [JSON] or [YAML] format.

```yaml
page:
  size: A4
header:
  content: '{title}'
  border-after: solid 1pt black
footer:
  content: '{folio}'
  text-align: center
style:
  body:
    font-family: Times New Roman
  topic:
    font-family: Arial
    font-size: 26pt
    font-weight: bold
    color: gray
```

[json]: https://json.org
[yaml]: https://yaml.org

The `com.elovirta.pdf` plug-in for DITA-OT extends the built-in PDF2 plug-in by adding support for the `theme` property. The `theme` property takes a path to a theme file and changes the styling of the PDF2 output without needing to extend the transtype with XSLT.

Themes can be used to adjust basic settings like cover page images, page sizes, numbering, font properties, background colors and borders, spacing, and running content like page headers and footers.

To generate PDF output with a custom theme, pass the path to the theme file to the `dita` command with the `--theme` option:

```shell
dita --project=samples/project-files/pdf.xml \
     --theme=/path/to/custom-theme-file.yaml
```

The following topics provide details on the theme file formats and supported configuration options.

## Theme file

A theme file can be written in either [JSON] or [YAML] format.

The examples provided here are all in YAML format, which is generally more compact and readable than JSON.

```yaml
page:
  size: A4
header:
  content: '{title}'
  border-after: solid 1pt black
footer:
  content: '{folio}'
  text-align: center
style:
  body:
    font-family: Times New Roman
  topic:
    font-family: Arial
    font-size: 26pt
    font-weight: bold
    color: gray
```

[json]: https://json.org
[yaml]: https://yaml.org

Add details on theme capabilities & YAML examples
Add details on theme capabilities & YAML examples

Signed-off-by: Roger Sheen <a61a1add71dadc74037ae5e44ee30713db9a7451@infotexture.net>
