fix(docs): fix invisible header text in dark mode

Override Material's var(--md-primary-bg-color) which resolves to near-black
in slate scheme, causing dark-on-dark header text.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/stylesheets/extra.css

@@ -89,6 +89,7 @@
 
 [data-md-color-scheme="slate"] .md-header {
   background-color: #161b22;
+  color: #f0f6fc;
 }
 
 /* Header title color fix */