Add a general a:link, a:visited dark mode rule so links anywhere in the
page (e.g. the description block) use the correct color instead of the
browser's default purple.
Add a:visited alongside a:link so visited links inherit the correct color
in both light and dark mode, instead of falling back to the browser's
default dark purple.
Introduces logo_dark and repo_icon_dark config options that serve alternate
images to dark-OS browsers via HTML <picture> elements, and adds a CSS
prefers-color-scheme dark block to the default header template.
