Respect the user agent property `prefers-color-scheme` by
rendering the documentation in dark mode, if requested.
Reorganise CSS to store the color palette into CSS variables
and reference these from another set of CSS variables that
are dynamically switched based on the prefers-color-scheme
media query.
Light mode variables stay they same as before,
but to ease color mixing, the current RGB values
have been transformed to HSL (using chrome devtools).
The current body background is now --sd-gray-extralight,
the current body color is now --sd-gray-extradark.
Other gray-variations, needed for dark-mode constrasts
are derived from these colors using HSL lightness shifting.
The systemd brand black color is used as dark mode background
and a matching extralight gray font color has been selected.
The link font-weight is reduced to 400 in dark mode,
as the green color on dark ground becomes to overwhelming
with a bold font.
The systemd logo color is dynamically swapped by using
the special fill value `currentColor` for dark/light-mode
depending parts – as per specification on brand.systemd.io
The theme color is used on android to style the chrome
browser-ui with a color that suits the webpage.
Use the dark brand color instead of a random default blue color
in order to fit to the rest of the color scheme.
Those were added in b41a3f66c9 without
an explicit license, so they are under the default license. Some files
already got a header previously, so this only touches the remaining.
The same should be done for docs/_data/extra_pages.json, but it's json, and
json doesn't allow comments.
