For now the best way to work on a theme is to run static-docs to produce the site:
$ make
Which produces the following files:
build
├── index.html
└── theme
└── apex
├── css
│ └── index.css
├── js
│ ├── index.js
│ └── smoothscroll.js
└── views
└── index.html
Open the site and edit anything you like.
$ open build/index.html
Copy and paste any changes back into ./docs/themes/apex.
This process will be improved at some point.