Add a dark mode

[SaschaWillems]

Been requested e.g. here: KhronosGroup/Vulkan-Tutorial#33 (Can't move the issue over here due to missing permissions)

[gpx1000]

KhronosGroup/antora-ui-khronos#11 <-- when that's merged it will display dark mode if your browser preferences are set to display dark mode.

[fluppeteer]

As mentioned on the call, other things to consider while offering configurable CSS:

Atkinson Hyperlegible was designed to maximise legibility for people who have visual limitations.

There are also some fonts like OpenDyslexic and Dyslexie which are intended to be easier to read for those with dyslexia. Supposedly reducing contrast can sometimes help there, although since the reverse is true of my vision and I had a load of low-contrast whiteboards imposed on me without consultation, probably options are the way in all things. At least we don't have a lot of animation to hide.

I'm not particularly a fan of the aesthetics of any of those, so I'm not advocating using them as a default, but it might be nice to have them as options?

(I'll be adding that functionality to a website I host soon, if I can report back on whether they work. Although actually the biggest problem I've had with a font choice is finding one whose fractions actually match the appearance of the main numerals, which is for some reason extremely rare, and very annoying if you're trying to make ¹⁄₁₀ look like ½.)

[gpx1000]

Thanks. I don't know if there's a preference we can take advantage of if the user has specified they prefer a specific font in their browser the way we do with darkmode. I think it's going to be a cookie or something along with a drop down to select the font; but it's certainly doable.

The only preference that I'm aware of is: @media (prefers-contrast:...) I think the normal usecase there is to increase/decrease the base-weight; however, we could change the default font?