Top-bar: use onSurfaceVariant for the gear icon (same fix as navbar)

Same root cause as the bottom navbar invisibility: iconColor was set to
theme.colorScheme.primary, which on the light theme is our dark forest
green (#1A5C38) and goes invisible on dark surfaces. onSurfaceVariant
is Material's de-emphasized-foreground token and has guaranteed contrast
against the surface across all our themes.

lib/new-ui/widgets/coins_page/top_bar_widget/top_bar.dart

@@ -45,7 +45,11 @@ class TopBar extends StatelessWidget {
               isSyncHeavy: dashboardViewModel.isSyncHeavy,
             ),
             ModernButton.svg(
-              iconColor: Theme.of(context).colorScheme.primary,
+              // Hash Wallet: was theme.colorScheme.primary (dark forest green)
+              // which rendered the gear icon nearly invisible on the dark
+              // dashboard backdrop. onSurfaceVariant is the de-emphasized-
+              // foreground Material token.
+              iconColor: Theme.of(context).colorScheme.onSurfaceVariant,
               size: 36,
               onPressed: () {
                 HapticFeedback.mediumImpact();