Extends Category Key

A theme can extend another theme using the extends key.

The extends key accepts either a single value or an array of values. Each value is interpreted as a filename. If the filename equals default, it resolves to the location of the default (built-in) theme. If the filename is absolute, it’s used as is. If the filename begins with ./, it’s resolved as a theme file relative to the current theme file. Otherwise, the filename is resolved as a theme file in the normal way (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 keys are overlaid onto the keys from the previous theme.