sidebar content wrongly styled in dark mode

[einarf]

Some documents use sidebar to display a notice in the document.

An example is /tutorials/raycasting/index

.. sidebar:: What is Shadertoy?

    Where does the name Shadertoy come from?
    This class is designed to mimic the `Shadertoy <https://www.shadertoy.com/>`_ website.
    The website makes it easy to experiment with shaders, and those shaders can be run
    using the Arcade library.

Some styling here is needed to make them look reasonable. Should not be too hard to fix. When extending the custom css make sure a comment explains why the tweaks exists or we will never figure out how to maintain is in the future.

[einarf]

Looks like this is just the .sidebar, .sidebar-title, ... classes. They need entries for data-theme="dark"

[einarf]

Made it readable. Can tweak later