docs site: legible header in dark mode, typography in its own stylesheet

Dark mode: the slate scheme set the header to #3f6b3e, a dark green on a dark page, so
the bar disappeared into the background. It is now green-mid with near-black text.

Typography moves to publishing/typography.css, loaded after brand.css, and declares
Figtree on the elements themselves -- Material defines --md-text-font-family on <body>,
so a :root value loses to it by DOM proximity. Heading colours are scoped to the light
scheme, since ink is unreadable on the dark one.

publishing/brand.css

@@ -48,7 +48,14 @@
 
 /* Dark scheme keeps the same accents; light stays the default. */
 [data-md-color-scheme="slate"] {
-  --md-primary-fg-color: #3f6b3e;
+  /* The header was #3f6b3e -- a dark green against a dark page, so the bar
+     disappeared into the background. Green-mid with near-black text keeps the
+     bar legible in dark mode while staying on brand. */
+  --md-primary-fg-color: #6ba86b;
+  --md-primary-fg-color--light: #7ab87a;
+  --md-primary-fg-color--dark: #5b8c5a;
+  --md-primary-bg-color: #16201a;
+  --md-primary-bg-color--light: rgba(22, 32, 26, 0.7);
   --md-accent-fg-color: #d98a75;
   --md-typeset-a-color: var(--green-light);
   --md-code-bg-color: #2e3328;
@@ -174,6 +181,11 @@ body,
   font-size: 2.6rem;
   font-weight: 700;
   letter-spacing: -0.02em;
+}
+/* ink is a light-scheme colour; in dark mode let Material's own fg colour apply */
+[data-md-color-scheme="default"] .md-typeset h1,
+[data-md-color-scheme="default"] .md-typeset h2,
+[data-md-color-scheme="default"] .md-typeset h3 {
   color: var(--ink);
 }
 
@@ -181,13 +193,11 @@ body,
   font-size: 1.7rem;
   font-weight: 600;
   letter-spacing: -0.01em;
-  color: var(--ink);
 }
 
 .md-typeset h3 {
   font-size: 1.25rem;
   font-weight: 600;
-  color: var(--ink);
 }
 
 .md-header__title {