Not able to use a custom theme / docs unclear on how to do it.

How exactly does one make a custom theme work: I tried initially to

- copy this folder locally in my project to docs/theme https://github.com/documentationjs/documentation/tree/master/src/default_theme
- run command
`documentation build --format html --theme docs/_theme --output docs/api/`

I get an error:
`/Users/tmcconechy/Dev/4.3.1/docs/_theme/index.js:22
comments: Array,
^
SyntaxError: Unexpected token :
at Object.exports.runInThisContext (vm.js:78:16)`

Tried beta 14 and now 18 to be sure. How does one succeed in getting it work with a theme?

Specifically i want to modify the markdown my usually command is:

`documentation build myfile.js -f md --theme docs/_theme -o myfile.md`

Theme is ignored in this scenario as well.

Was hoping if i point it at a theme it would use that before generating the MD as per https://github.com/documentationjs/documentation/blob/master/docs/THEMING.md