# Extending themes

A theme can extend another theme using the `extends` key.

If the value is `default`, it resolves to the built-in default theme. Otherwise the value of `extends` is a relative path from the current theme file to the theme being extended. If a theme doesn't have an `extends` key, default PDF2 plug-in styles are used.

`base.yaml`

    brand:
      primary-color: orange
    page:
      size: A4

`product/theme.yaml`

    extends: ../base.yaml
    page:
      size: Letter
    style:
      topic:
        color: $brand-primary-color