Add dark mode support to the HTML versions of the OpenAPI specs

[mikekistler]

Update the rendering process for the HTML versions of the OpenAPI specs (using ReSpec) to generate pages that support dark mode.

ReSpec has added dark mode support: speced/respec#2651

Related: #3576

[handrews]

Paging @Bellangelo (we're working on making it so that we can assign issues to you, but haven't quite sorted that out yet).

[Bellangelo]

Unfortunately, this seems not to be so easy. Our js/respec-oai.js file loads this css file https://www.w3.org/StyleSheets/TR/2016/base.css which overrides most styles. There is a https://www.w3.org.org/StyleSheets/TR/2026/dark.css but even if we add it, it still gets overridden by the base.css.
It looks like we need to give more effort in this issue #3845 which will help us have more control over the respec-oai.js

[ralfhandl]

ReSpec documentation of dark mode says

Note: This is supported for W3C specs only, and makes use of official dark.css in tr-design.

We can enable the switch, and it has no effect 😞

[ralfhandl]

@mikekistler Don't know what changed in the last two weeks, now dark mode seems to work (at least if your system theme is set to dark mode):

• https://ralfhandl.github.io/OpenAPI-Specification/oas/latest-dark.html

Quite some fine-tuning needed for dark mode to really work:

• different OAS logos for light/dark mode
• table and code-block background color depending on light/dark mode
• ...

I'll gladly leave this to someone fluent in CSS 😁

[pavelkornev]

I have started on this, please assign to my name.

Intermediate results can be seen here: https://pavelkornev.github.io/OpenAPI-Specification/oas/latest.html