add background color for night mode

mobile/lib/presentation/widgets/asset_viewer/activities_bottom_sheet.widget.dart

@@ -79,7 +79,7 @@ class ActivitiesBottomSheet extends HookConsumerWidget {
       expand: false,
       shouldCloseOnMinExtent: false,
       resizeOnScroll: false,
-      backgroundColor: context.isDarkTheme ? Colors.black : Colors.white,
+      backgroundColor: context.isDarkTheme ? context.colorScheme.surfaceContainer : Colors.white,
     );
   }
 }

mobile/lib/presentation/widgets/asset_viewer/asset_viewer.page.dart

@@ -696,7 +696,7 @@ class _AssetViewerState extends ConsumerState<AssetViewer> {
               enablePanAlways: true,
             ),
             if (!showingBottomSheet)
-              Positioned(
+              const Positioned(
                 bottom: 0,
                 left: 0,
                 right: 0,
@@ -704,7 +704,7 @@ class _AssetViewerState extends ConsumerState<AssetViewer> {
                   mainAxisSize: MainAxisSize.min,
                   mainAxisAlignment: MainAxisAlignment.end,
                   crossAxisAlignment: CrossAxisAlignment.stretch,
-                  children: [const AssetStackRow(), const ViewerBottomBar()],
+                  children: [AssetStackRow(), ViewerBottomBar()],
                 ),
               ),
           ],

mobile/lib/presentation/widgets/asset_viewer/bottom_sheet.widget.dart

@@ -79,7 +79,7 @@ class AssetDetailBottomSheet extends ConsumerWidget {
       expand: false,
       shouldCloseOnMinExtent: false,
       resizeOnScroll: false,
-      backgroundColor: context.isDarkTheme ? Colors.black : Colors.white,
+      backgroundColor: context.isDarkTheme ? context.colorScheme.surfaceContainer : Colors.white,
     );
   }
 }