Adds dark theme and theme switching functionality (REPL + website)

[agriyakhetarpal]

Description

This PR adds a dark theme to the overall website. The REPL iframe can switch themes without having to restart the kernel, which requires a theme bridge as described in https://jupyterlite.readthedocs.io/en/stable/howto/configure/advanced/iframe.html

Tip

This PR is a work-in-progress and is not yet ready for review. However, initial feedback is welcome. The description below is a stub.

An initial look is looking good, so far:

Screen.Recording.2025-01-16.at.2.43.37.AM.mov

Closes sympy/sympy.github.com#199

TODO items

• Get the theme switch toggle button placed in a proper position
  • Increase sizing for button text
  • Decide position: whether to add to the top navbar or to the top right of the REPL section
  • Decide whether to use FontAwesome icons (PyData Sphinx Theme) v.s. sticking with ☀️🌗🌙 emoji
• Move the theme bridge switch to a separate repository and package it, perhaps?
  • Contribute to the JupyterLite docs upstream to respect prefers-color-scheme patch
  • Ask JupyterLite developers what the best solution would be for this
• Conduct an accessibility audit for dark mode support
  • Resolve known issues
    • SymPy text for <768px in dark green header should be white and not light green
    • Check if mobile theming works
• Add instructions for the dark theme bridge in the README

[jtpio]

This looks great, thanks @agriyakhetarpal!

Move the theme bridge switch to a separate repository and package it, perhaps?

For reference there is now an extension to provide that kind of bridge between a host page and a JupyterLab / JupyterLite instance running in the IFrame: https://github.com/TileDB-Inc/jupyter-iframe-commands.

So it could be interesting to try to use that extension here, and see if it helps simplify the diff and not have to add a new extension. That would mean:

• installing the jupyter-iframe-commands extension in the JupyterLite environment
• using the jupyter-iframe-commands-host package to send the command for switching theme