fix: declare color-scheme, so navigation stops flashing white

The mark is fill="currentColor" inside an anchor, so an unstyled paint renders
it in the UA link blue. That is what the flash is: the document painted before
the stylesheet applies, not a theme switch.

Without a color-scheme declaration the browser treats the document as light, so
the canvas it paints between documents, and the scrollbar, are white. Gecko
paints a pending stylesheet's document far more readily than Blink does, which
is why it shows there and not in Chromium.

Dark on flux and bbs, which are dark only; all three branches on raayoni.

The other half of this is a Caddy change in bucur: every response currently
carries no Cache-Control, no ETag and no Last-Modified, so the stylesheet is
refetched on every navigation and the window keeps reopening.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

randogoth.com/bbs/static/bbs.css

@@ -17,6 +17,9 @@
 
 /* the IBM EGA/VGA text palette, unaltered */
 :root {
+  /* A board is black by definition; tell the browser so the canvas between
+     documents and the scrollbar match instead of flashing light. */
+  color-scheme: dark;
   --black:   #000000;  --blue:     #0000aa;  --green:   #00aa00;  --cyan:    #00aaaa;
   --red:     #aa0000;  --magenta:  #aa00aa;  --brown:   #aa5500;  --grey:    #aaaaaa;
   --dgrey:   #555555;  --lblue:    #5555ff;  --lgreen:  #55ff55;  --lcyan:   #55ffff;