feat(ground-suite): add theme selector

docs/src/ground-suite.md

@@ -16,6 +16,8 @@ or write capability.
 Ground Suite provides light and dark themes, selecting the operating system's
 current preference by default. The dark theme uses near-black, muted-silver,
 and restrained green accents with locally blurred translucent panels.
+The observation bar also offers a current-session System, Light, or Dark
+appearance selector.
 
 Ground Suite uses a receive-only source/decoder/classifier boundary. The
 replay implementation feeds typed observations today. A future live adapter