automatically set the right theme according to prefers-color-scheme

[tleydxdy]

https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-color-scheme

currently the theme only work when JS is enabled it seems

[h-enk]

Will be implemented soon (including no FART) — implemented yesterday on getdoks.org

[tleydxdy]

I tried out getdoks.org, still doesn't seem to be working. (without JS at least)