custom theme extended from alabaster or sphinx_rtd_theme can't find base theme.

[shimizukawa]

Sphinx 1.3 include theme paths for 'alabaster' and 'sphinx_rtd_theme' automatically if html_theme is specified them.
However, if user custom theme extends from them, the path isn't included automatically.

IMO, a custom theme has inherit = alabaster or inherit = sphinx_rtd_theme, the base theme path should be included automatically.

[shimizukawa]

Now sphinx theme loader loads alabaster and sphinx_rtd_theme automatically if a theme is inherited from them.This ticket can be closed.

However, alabaster and sphinx_rtd_theme doesn't have theme name directory.
Consequence of it, layout.html of custom derived theme can't extend from them because {%- extends "alabaster/layout.html" %} is not valid and {%- extends "!layout.html" %} cause infinite loop (leadning exclamation mark excludes _templates, but doesn't exclude custom theme directory itself).

In conclusion, themes without a theme name directory in the theme path (like alabaster-0.7.3 and sphinx_rtd_theme-0.1.7) is not extendable.

[shimizukawa]

However, alabaster and sphinx_rtd_theme doesn't have theme name directory.
Consequence of it, layout.html of custom derived theme can't extend from them because {%- extends "alabaster/layout.html" %} is not valid and {%- extends "!layout.html" %} cause infinite loop (leadning exclamation mark excludes _templates, but doesn't exclude custom theme directory itself).

In conclusion, themes without a theme name directory in the theme path (like alabaster-0.7.3 and sphinx_rtd_theme-0.1.7) is not extendable.

I was wrong.
Now sphinx supports theme that inherit from non-builtin themes.