HTML Theme for all asciidoc files
Add role=theme-swap (AsciiDoc) or class="theme-swap" (HTML) to an image and
provide a dark variant named <name>-dark.<ext> next to it. In dark mode the
theme loads the dark variant automatically.
This works for link-based image builds only. Curriculum HTML built with
-a data-uri inlines images as base64, so there is no filename to rewrite and
the swap is a no-op.