+... customize the look of the built HTML files?
+ Use themes, see :doc:`theming`.
... add global substitutions?
Add them in the :confval:`rst_epilog` config value.
+... write my own extension?
+ See the :ref:`extension tutorial <exttut>`.
... use Sphinx with Epydoc?
There's a third-party extension providing an `api role`_ which refers to
Epydoc's API docs for a given identifier.