Unhelpful error message when theme.conf is missing

[vsajip]

Describe the bug

If the theme.conf file is missing, the code here:

sphinx/sphinx/theming.py

Line 72 in fcc3899

self.config.read(path.join(self.themedir, THEMECONF), encoding='utf-8')

creates an empty configuration and fails a few lines later at

sphinx/sphinx/theming.py

Line 77 in fcc3899

raise ThemeError(__('theme %r doesn\'t have "theme" setting') % name) from exc

because of course there's no [theme] section, as the file doesn't exist!

It would be more helpful to raise an error saying that the theme.conf file is missing.

How to Reproduce

Just remove theme.conf from a theme and try to build some documentation with that theme.

Environment Information

Platform:              linux; (Linux-4.15.0-213-generic-x86_64-with-glibc2.27)
Python version:        3.10.8 (main, Nov 24 2022, 07:33:30) [GCC 7.5.0])
Python implementation: CPython
Sphinx version:        7.2.5
Docutils version:      0.20.1
Jinja2 version:        3.1.2
Pygments version:      2.16.1

Sphinx extensions

None

Additional context

No response

[picnixz]

PR is welcomed.

You can first check the existence of the file and raise a warning accordingly. (The current check on the 'theme' setting should still be done)

If no one submits a PR, I'll have a look at it upon my return (late September).