theme: override --presentation and --surface-chat (chat-form backdrop + messages area were still white in light mode)

assets/custom.css

@@ -23,6 +23,9 @@ html.light {
   --text-primary: 45 51 39;         /* --ink #2d3327 */
   --text-secondary: 107 122 98;     /* --muted #6b7a62 */
   --border-light: 205 224 203;      /* green-tinted border */
+  --presentation: 234 243 234;      /* chat-form backdrop (was white) */
+  --surface-chat: 234 243 234;      /* messages area (was white) */
+  --surface-composer-hover: 223 237 222;
 }
 
 html.dark {
@@ -39,6 +42,9 @@ html.dark {
   --text-primary: 232 245 232;      /* #e8f5e8 */
   --text-secondary: 148 160 140;
   --border-light: 74 82 64;
+  --presentation: 34 39 31;
+  --surface-chat: 34 39 31;
+  --surface-composer-hover: 42 48 39;
 }
 
 /* ---- Figtree font (design kit, SIL OFL) — matches homepage & dashboard ---- */