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/typography.css

@@ -0,0 +1,46 @@
+/* Typography for the co-op documentation site.
+ *
+ * Loaded after publishing/brand.css on purpose: the font has to be declared on the
+ * elements themselves. Material renders body and headings with
+ * var(--md-text-font-family), which it defines on <body>, so a :root-level value
+ * loses to it by DOM proximity no matter how the cascade is ordered.
+ *
+ * Figtree is self-hosted -- see the @font-face rules in brand.css. No CDN, no
+ * third-party font service.
+ */
+
+body,
+.md-typeset,
+.md-nav,
+.md-header,
+.md-tabs,
+.md-footer {
+  font-family: "Figtree", system-ui, -apple-system, "Segoe UI", Roboto,
+    "Helvetica Neue", Arial, sans-serif;
+}
+
+.md-typeset h1,
+.md-typeset h2,
+.md-typeset h3,
+.md-typeset h4,
+.md-typeset h5,
+.md-typeset h6,
+.md-header__title,
+.md-nav__title {
+  font-family: "Figtree", system-ui, -apple-system, "Segoe UI", Roboto,
+    "Helvetica Neue", Arial, sans-serif;
+  letter-spacing: -0.01em;
+}
+
+.md-typeset h1 { font-size: 2.6rem; font-weight: 700; letter-spacing: -0.02em; }
+.md-typeset h2 { font-size: 1.7rem; font-weight: 600; }
+.md-typeset h3 { font-size: 1.25rem; font-weight: 600; }
+.md-header__title { font-weight: 600; }
+
+/* Colour is scheme-specific, so it stays out of the rules above. */
+[data-md-color-scheme="default"] .md-typeset h1,
+[data-md-color-scheme="default"] .md-typeset h2,
+[data-md-color-scheme="default"] .md-typeset h3,
+[data-md-color-scheme="default"] .md-typeset h4 {
+  color: #2d3327;
+}