Yes, the documentation is outdated. The parameter ResourceDir was added in OpenACS 5.9.0, in OpenACS 5.9.1 the theme manager [1,2] as added. Styling is currently best described in [3].
A theme can be defined per subsite (and be reused on several subsites). The theme can overload all templates that it whishes to refine. See e.g. the files overloaded my the openacs-bootstrap3-theme
find packages/openacs-bootstrap3-theme/ -type f|fgrep -v CVS
What about the widget tag?
Not sure, who introduced the "widget" tag at which time, probably one of the elders. It was used on various places in the openacs site. If i remember correctly, in OpenACS 5.9, it was added to the general code to reduce local differences. The "widget" tag is like the "include" tag, but it its searched additionally in a different directory. Examples for the openacs site are the recent announcements, recent postings, recent news, recent bugs, ... hosting, companies, etc.
So, widgets are essentially site-specific includes kept at a special place to avoid inferences with upstream changes. Its name probably comes from [4].
The place, where the "widgets" are kept (under the theme) is not perfect, probably this would be better placed under acs-subsite (would be usable for every subsite and every theme). The "widget" predates the theme-specific overloading of "includes".
[1] https://openacs.org/xowiki/openacs-todo
[2] https://openacs.org/Announce-OpenACS-5.9.1
[3] https://openacs.org/xowiki/oacs-theming
[4] https://openacs.org/doc/acs-templating/guide/composite
