www: single #227842 brand green for λ mark, period, links, CTAs

Collapse --accent (was blue #0b5394) into the same green as the
period. One color now marks everything that says "lumbda": the λ
logo, the declarative period, link underlines, and the CTA buttons.
Dark-mode variant lightened to #5ec07a so the green reads on
#161613.

www/style.css

@@ -13,8 +13,8 @@
   --fg: #1a1a1a;
   --bg: #fafaf7;
   --muted: #666;
-  --accent: #0b5394;
-  --green: #2a8a3a;   /* growth — colors the declarative period */
+  --accent: #227842;   /* single brand green — λ mark, period, links, CTAs */
+  --green: #227842;
   --rule: #d4d4d0;
   --code-bg: #f0ede4;
 }
@@ -24,8 +24,8 @@
     --fg: #e8e6e0;
     --bg: #161613;
     --muted: #9a9892;
-    --accent: #6fa8dc;
-    --green: #6fd68a;
+    --accent: #5ec07a;   /* lightened for contrast on dark bg */
+    --green: #5ec07a;
     --rule: #3a3a36;
     --code-bg: #22221f;
   }
@@ -69,7 +69,7 @@ header .lambda-mark {
   font-family: 'chunkfive', Georgia, serif;
   font-size: 5rem;
   line-height: 0.9;
-  color: var(--fg);
+  color: var(--green);
 }
 
 header h1 {