Furo dark theme does not work

[bbhtt]

This issue happens when trying to build xdg-desktop-portal documentation.

When installing furo and sphinx via pip, which fetches the latest version, the theme seems to work fine. I believe this is due to an incompatibility between the sphinx and furo versions available in Ubuntu Noble repositories and updating both should solve it.

Steps to reproduce:

1. Create a container/system with Ubuntu Noble
2. `git clone <email address hidden>:flatpak/xdg-desktop-portal.git && cd xdg-desktop-portal`
3. Install the dependencies via apt from Containerfile https://github.com/flatpak/xdg-desktop-portal/blob/ddef51ea19f35ba532e3a8027929363e5d3ae0a7/.github/workflows/Containerfile#L11-L50
4. Install doc dependencies from Ubuntu repositores: `sudo apt install -y --no-install-recommends python3-sphinx-copybutton python3-sphinxext-opengraph furo python3-sphinx`
5. Compile the docs: `meson setup builddir -Ddocumentation=enabled && ninja -C builddir doc/html`
6. Run http.server and open on browser: `python3 -m http.server -d builddir/doc/html & xdg-open http://localhost:8000`
7. See that the theme does not switch automatically to dark mode based on OS/browser setting and the copybuttons and some other CSS are messed up. (See attached image)

Versions:

python3-sphinx-copybutton (0.4.0-5).
python3-sphinxext-opengraph (0.7.5-1).
furo (2024.01.29+dfsg-1).
python3-sphinx (7.2.6-6).

lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 24.04.1 LTS
Release: 24.04
Codename: noble