changed graph line color to primary

lib/widgets/reading_graph.dart

@@ -27,7 +27,7 @@ class ReadingGraph extends StatelessWidget {
         lineBarsData: [
           LineChartBarData(
             isCurved: true,
-            color: Colors.cyan,
+            color: Theme.of(context).colorScheme.primary,
             barWidth: 2,
             isStrokeCapRound: true,
             dotData: const FlDotData(show: false),