gps related minor fixes

pi/ui/lib/widgets/gps_compass.dart

@@ -61,7 +61,7 @@ class GpsCompass extends StatelessWidget {
               _hasSignal ? "${_displayHeading} ${_compassDirection}" : (_isAcquiring ? "ACQ" : "N/A"),
               style: TextStyle(
                 fontSize: 80,
-                color: theme.subdued,
+                color: theme.subdued, // less emphasis on text, let the icon have semantic colour
                 fontFamily: 'DIN1451',
               ),
             ),

pi/ui/lib/widgets/system_bar.dart

@@ -70,7 +70,7 @@ class SystemBar extends StatelessWidget {
                   value: gpsState == 'acquiring' ? 'ACQ'
                        : gpsState == 'fix' ? (gpsSatellites?.toString() ?? 'N/A')
                        : '0',  // lost or unknown
-                  isAbnormal: gpsState != 'fix',
+                  isAbnormal: gpsState != 'fix' || gpsSatellites == null,
                   alignment: Alignment.centerLeft,
                   labelSize: labelSize,
                   valueSize: valueSize,