added default theme

lib/widgets/reading_graph.dart

@@ -48,12 +48,14 @@ class ReadingGraph extends StatelessWidget {
         ),
         lineTouchData: LineTouchData(
           touchTooltipData: LineTouchTooltipData(
+            getTooltipColor: (touchedSpot) =>
+                Theme.of(context).colorScheme.primaryContainer,
             getTooltipItems: (touchedSpots) {
               return touchedSpots.map((touchedSpot) {
                 return LineTooltipItem(
                     '${touchedSpot.y.toString()}\n${_getLongDate(touchedSpot.x)}',
                     TextStyle(
-                        color: Theme.of(context).colorScheme.onPrimary,
+                        color: Theme.of(context).colorScheme.onPrimaryContainer,
                         fontWeight: FontWeight.bold));
               }).toList();
             },