docs site: use Material's custom palette so the co-op colours actually apply

The theme was configured with primary: teal and accent: deep orange, so every page
shipped data-md-color-primary="teal" / data-md-color-accent="deep-orange" and Material
set --md-primary-fg-color and friends on <body>. Those variables then beat brand.css,
which sets them on <html>, because a value on the nearer ancestor wins for descendants
regardless of specificity. The result: cream background and ink from brand.css applied,
but the header, links and accents stayed Material's defaults.

primary/accent are now "custom", Material emits no palette of its own, and the co-op
variables in publishing/brand.css take effect.

mkdocs.yml

@@ -40,15 +40,15 @@ theme:
     # Light is listed first, so a fresh visitor gets the bright theme.
     - media: "(prefers-color-scheme: light)"
       scheme: default
-      primary: teal
-      accent: deep orange
+      primary: custom
+      accent: custom
       toggle:
         icon: material/weather-night
         name: Switch to dark mode
     - media: "(prefers-color-scheme: dark)"
       scheme: slate
-      primary: teal
-      accent: deep orange
+      primary: custom
+      accent: custom
       toggle:
         icon: material/weather-sunny
         name: Switch to light mode