feat(ground-suite): add theme selector

ground-suite/README.md

@@ -52,6 +52,9 @@ by default. Its dark palette uses near-black, muted-silver, and restrained
 green accents; translucent, locally blurred panels keep operational content
 legible without turning diagnostics into decoration.
 
+Use the appearance button in the observation bar to select **System default**,
+**Light**, or **Dark**. The choice applies for the current session only.
+
 ## Deliberately deferred work
 
 - TODO(gcs-live-mavlink): generated MAVLink Common plus custom-dialect