Allow configuring theme color

[shuding]

Currently we have a --nextra-primary-hue variable so you can adjust the global theme color. I think this can be a good theme configuration, we accept a number and render a <style> tag in <NextHead> that contains this hue variable. At the same time we can have another hue for the dark mode too:

primaryHue?: number | {
  light?: number
  dark?: number
}

Originally posted by @alii in #445 (comment)