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. 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.