Problem

When trying to use sphinx_rtd_theme==3.0.2 with Sphinx==9.0.0, pip refuses to resolve dependencies because the theme currently pins Sphinx to <9:

sphinx-rtd-theme 3.0.2 depends on sphinx<9 and >=6
The user requested sphinx==9.0.0

Since Sphinx 9.0.0 is now released and installable, this strict upper bound prevents using sphinx_rtd_theme with the latest Sphinx, even for simple projects that otherwise work fine.

I’m filing this to request:

1. Support for Sphinx 9.0.0 in sphinx_rtd_theme, and
2. A relaxed upper bound in install_requires (e.g. sphinx<10) once compatibility is confirmed.

Reproducible Project

This is a pure dependency-resolution issue and does not depend on any particular RST content. A minimal repro is:

python3 -m venv venv
source venv/bin/activate

python3 -m pip install --upgrade pip

python3 -m pip install \
  "sphinx==9.0.0" \
  "sphinx_rtd_theme==3.0.2" \
  "nbsphinx"

Error Logs/Results

Relevant excerpt from pip output:

Requirement already satisfied: sphinx==9.0.0 in ./venv/lib/python3.13/site-packages (9.0.0)
Collecting sphinx_rtd_theme==3.0.2
  Using cached sphinx_rtd_theme-3.0.2-py2.py3-none-any.whl.metadata (4.4 kB)
INFO: pip is looking at multiple versions of sphinx-rtd-theme to determine which version is compatible with other requirements. This could take a while.
Collecting sphinx==9.0.0
  Using cached sphinx-9.0.0-py3-none-any.whl.metadata (5.8 kB)
ERROR: Cannot install sphinx-rtd-theme==3.0.2 and sphinx==9.0.0 because these package versions have conflicting dependencies.

The conflict is caused by:
    The user requested sphinx==9.0.0
    sphinx-rtd-theme 3.0.2 depends on sphinx<9 and >=6

ERROR: ResolutionImpossible: for help visit https://pip.pypa.io/en/latest/topics/dependency-resolution/#dealing-with-dependency-conflicts

Expected Results

Once sphinx_rtd_theme is compatible with Sphinx 9, I would expect:

• pip install "sphinx==9.0.0" "sphinx_rtd_theme==<new-version>" to succeed without dependency conflicts.
• The theme to work as usual under Sphinx 9.0.0 (basic builds, HTML output, etc.).

Environment Info

• Python Version: 3.13.9
• Sphinx Version: 9.0.0
• RTD Theme Version: 3.0.2