Since Hugo 0.42 a project can configure a theme as a composite of as many theme components you need:
You can even nest this, and have the theme component itself include theme components in its own config.toml (theme inheritance).1
The theme definition example above in config.toml creates a theme with 3 theme components with precedence from left to right.
For any given file, data entry, etc., Hugo will look first in the project and then in my-shortcode, base-theme, and lastly hyde.
Hugo uses two different algorithms to merge the filesystems, depending on the file type:
For i18n and data files, Hugo merges deeply using the translation id and data key inside the files.
For static, layouts (templates), and archetypes files, these are merged on file level. So the left-most file will be chosen.
The name used in the theme definition above must match a folder in /your-site/themes, e.g. /your-site/themes/my-shortcodes. There are plans to improve on this and get a URL scheme so this can be resolved automatically.
Also note that a component that is part of a theme can have its own configuration file, e.g. config.toml. There are currently some restrictions to what a theme component can configure:
params (global and per language)
menu (global and per language)
outputformats and mediatypes
The same rules apply here: The left-most param/menu etc. with the same ID will win. There are some hidden and experimental namespace support in the above, which we will work to improve in the future, but theme authors are encouraged to create their own namespaces to avoid naming conflicts.
Including theme components in the themes is currently not supported for themes hosted on The Hugo Themes Site, but can be really useful if you want to create your own theme based on a theme you find on that site.