What's up with sphinx/themes/pyramid/static/epub.css?

It has a header that claims that it's default.css_t:

https://github.com/sphinx-doc/sphinx/blob/302c23fb5c172826499e8b3fb6d9eccd3c32d307/sphinx/themes/pyramid/static/epub.css#L1-L10

In reality, this file is very similar to basic.css_t, it has only a few differences:

https://github.com/sphinx-doc/sphinx/blob/302c23fb5c172826499e8b3fb6d9eccd3c32d307/sphinx/themes/basic/static/basic.css_t

If both files are actually needed, shouldn't they re-use the common part?

The actual file default.css_t doesn't seem to exist, yet it is mentioned in the code:

https://github.com/sphinx-doc/sphinx/blob/af62fa61e6cbd88d0798963211e73e5ba0d55e6d/sphinx/builders/changes.py#L146-L147

There is only a default.css, which is clearly different:

https://github.com/sphinx-doc/sphinx/blob/302c23fb5c172826499e8b3fb6d9eccd3c32d307/sphinx/themes/default/static/default.css

default.css is also mentioned here, which also seems out of place:

https://github.com/sphinx-doc/sphinx/blob/26f620253dbbac98f3472ac5843d2d8f1bcb2726/sphinx/builders/html/__init__.py#L464-L469

It seems to have something to do with the "changes" builder (which I've never heard of before today):

https://github.com/sphinx-doc/sphinx/blob/302c23fb5c172826499e8b3fb6d9eccd3c32d307/sphinx/themes/basic/changes/versionchanges.html#L10

I can't really check what that's all about, because the "changes" builder doesn't seem to work for me (it creates an empty output directory).

Finally, default.css is mentioned in a few places regarding configuration, where it's probably also a bit anachronistic:

https://github.com/sphinx-doc/sphinx/blob/af62fa61e6cbd88d0798963211e73e5ba0d55e6d/doc/_static/conf.py.txt#L151-L154

https://github.com/sphinx-doc/sphinx/blame/d47427e94fee59f5a76f4585dac5ed9a08383a59/doc/usage/configuration.rst#L993-L998

https://github.com/sphinx-doc/sphinx/blob/af62fa61e6cbd88d0798963211e73e5ba0d55e6d/sphinx/templates/quickstart/conf.py_t#L94-L97

What's up with all that?
Am I missing something?

Start by comparing sphinx/themes/pyramid/static/epub.css_t with sphinx/themes/basic/static/basic.css_t, then trace the default.css_t and default.css references in sphinx/builders/changes.py, sphinx/builders/html/init.py, and the configuration templates. Check how the changes builder and referenced assets are exercised. Done means the stylesheet references and shared CSS are consistent, obsolete names are resolved, and the changes builder still produces its intended output.