Add support for dark mode

[wborn]

At night the website (especially documentation) can be blinding to the eyes. Maybe we can also support dark mode?

Such a user preference can be detected using the prefers-color-scheme user preference media query. Detection is currently supported by ~76% of all browsers used world wide (see caniuse). For some examples see also MDN.

[Confectrician]

I have found https://github.com/tolking/vuepress-theme-default-prefers-color-scheme which seems to add the prefers-colors-scheme mediea query to the default theme.
Also this is discussed in an vuepress issue already.
Ref:
vuejs/vuepress#1865

[ghys]

This plugin is for VuePress 1.x ;)
We're still on 0.x, 1.x has quite a few differences, so there should be some breaking changes to expect and we'll have to be careful if/when we migrate.

[Confectrician]

I think we had some more issues where 1.x was topic.
We should maybe think about doing an update anyways in the nearer future.

[wborn]

I see https://next.openhab.org/ now has a proper dark mode so this issue should be fixed soon. 😎