Apply UGA brand visual identity and accessibility improvements

Restyle site to align with brand.uga.edu guidelines: Bulldog Red primary,
Olympic/Lake Herrick accents, Oswald headings, Merriweather Sans body text.
Add WCAG AA-compliant dark mode, skip-to-content link, focus indicators,
and underlined content links for Section 508 compliance.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

mkdocs.yml

@@ -4,16 +4,17 @@ site_url: https://docs.gtbop.com/
 
 theme:
   name: material
+  custom_dir: overrides
   palette:
     - scheme: default
-      primary: deep green
-      accent: amber
+      primary: custom
+      accent: custom
       toggle:
         icon: material/brightness-7
         name: Switch to dark mode
     - scheme: slate
-      primary: deep green
-      accent: amber
+      primary: custom
+      accent: custom
       toggle:
         icon: material/brightness-4
         name: Switch to light mode
@@ -30,7 +31,7 @@ theme:
   icon:
     logo: material/bug
   font:
-    text: Source Sans Pro
+    text: Merriweather Sans
     code: Source Code Pro
 
 plugins:
@@ -53,6 +54,9 @@ markdown_extensions:
       emoji_index: !!python/name:material.extensions.emoji.twemoji
       emoji_generator: !!python/name:material.extensions.emoji.to_svg
 
+extra_css:
+  - stylesheets/uga-brand.css
+
 extra:
   social:
     - icon: fontawesome/solid/building-columns