Extends Category Key

A theme can extend another theme using the extends key.

The extends key should always be the first key in the theme file. The key accepts either a single value or an array of values. Each value is interpreted as a filename with an implicit -theme.yml suffix if not already present.

If the value is default, it resolves to the filename of the default (built-in) theme. If the value is an absolute path, it’s used as is. If the value begins with ./, it’s always resolved to a theme file relative to the current theme file. Otherwise, the filename is resolved relative to the value of the pdf-themesdir attribute.

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