Say how the GitHub mirror shows the wordmark

README.md

@@ -4,10 +4,10 @@
      browser is set - so anything conditional shows dark ink on a dark page to
      a reader whose browser prefers light, and does it silently.
      The suffix names the mode a file is FOR, not the colour of its ink:
-     -darkmode is the pale wordmark that belongs on a dark page. A mirror on a
-     light-themed host swaps the filename below for dejima-wordmark-lightmode.png,
-     which is committed alongside for exactly that. One line, on the mirror,
-     where someone can see whether it worked.
+     -darkmode is the pale wordmark that belongs on a dark page. GitHub does
+     follow its own theme setting through <picture>, so the GitHub mirror
+     replaces the tag below with one that falls back to
+     dejima-wordmark-lightmode.png; see dejima's mirror/github-mirror.sh.
      Every &nbsp; here is spacing, and it has to be a character rather than CSS
      or a presentation attribute. Gitea's sanitizer strips `style` from the tag
      and from any wrapper, `hspace`/`vspace` survive sanitizing only to be

assets/README.md

@@ -5,9 +5,9 @@ page that renders dark, `-lightmode` for one that renders light. The suffix
 names the mode a file is for, not the colour of its ink, so `-darkmode` is the
 pale one.
 
-The README points at `-darkmode` unconditionally. A mirror on a light-themed
-host changes that one line; the comment above the tag says why it is not
-conditional.
+The README points at `-darkmode` unconditionally; the comment above the tag
+says why it is not conditional. The GitHub mirror rewrites that tag to pick
+between the two by the reader's theme.
 
 **These are trademarks and are not covered by this repository's GPL licence.**
 Who holds them, and what may be done with them, is stated once for the whole

test/links.bats

@@ -96,8 +96,8 @@ anchors_of() {
              | sed 's/^src="//; s/"$//; s/^](//; s/)$//')
     [ "$found" -ge 2 ] || { echo "expected at least the wordmark and the badge"; false; }
 
-    # Both wordmark modes ship, so a mirror on a light-themed host changes one
-    # line rather than needing a file it does not have.
+    # Both wordmark modes ship, because the GitHub mirror picks between them
+    # by the reader's theme.
     [ -f "$REPO/assets/dejima-wordmark-darkmode.png" ]
     [ -f "$REPO/assets/dejima-wordmark-lightmode.png" ]
 }