theme handbook to match rethink Gitea + rename to 'a rethink development'

Mirror the rethink Gitea theme (data-theme=rethink) on the docs site:
- docs/stylesheets/extra.css maps the exact palette onto Material's slate
  scheme: body #0a0f1f, nav #061541, text #eef1f6, primary #569bcc,
  accent #55bbff, borders #294274; square corners and blue card/table
  borders echoing the Gitea repo panels.
- Force dark-only (drop the light/dark toggle; Gitea is dark-only).
- Rename site to lowercase 'a rethink development' (site_name, copyright,
  landing H1).

Verified: mkdocs build --strict clean; rendered landing + content pages
in-browser against the live Gitea page — colors, header bar, links, and
cards match.

Signed-off-by: disqualifier <dev@disqualifier.me>

mkdocs.yml

@@ -1,26 +1,18 @@
-site_name: Rethink Studios Handbook
+site_name: a rethink development
 site_description: Libraries, coding standards, and how to deploy on our network.
 site_url: https://docs.rethinkstudios.io/
-copyright: Rethink Studios
+copyright: rethink development
+
+extra_css:
+  - stylesheets/extra.css
 
 theme:
   name: material
   language: en
   palette:
-    - media: "(prefers-color-scheme: light)"
-      scheme: default
-      primary: indigo
-      accent: indigo
-      toggle:
-        icon: material/weather-night
-        name: Switch to dark mode
-    - media: "(prefers-color-scheme: dark)"
-      scheme: slate
-      primary: indigo
-      accent: indigo
-      toggle:
-        icon: material/weather-sunny
-        name: Switch to light mode
+    scheme: slate
+    primary: custom
+    accent: custom
   features:
     - navigation.instant
     - navigation.tracking