Theming: chart use default design background color.

src/main/java/de/dennisguse/opentracks/chart/ChartView.java

@@ -19,7 +19,6 @@ package de.dennisguse.opentracks.chart;
 import android.content.Context;
 import android.content.Intent;
 import android.graphics.Canvas;
-import android.graphics.Color;
 import android.graphics.DashPathEffect;
 import android.graphics.Paint;
 import android.graphics.Paint.Align;
@@ -47,6 +46,7 @@ import de.dennisguse.opentracks.stats.ExtremityMonitor;
 import de.dennisguse.opentracks.util.IntentUtils;
 import de.dennisguse.opentracks.util.MarkerUtils;
 import de.dennisguse.opentracks.util.StringUtils;
+import de.dennisguse.opentracks.util.ThemeUtils;
 import de.dennisguse.opentracks.util.UnitConversions;
 
 /**
@@ -89,6 +89,7 @@ public class ChartView extends View {
     private final List<ChartPoint> chartPoints = new ArrayList<>();
     private final List<Marker> waypoints = new ArrayList<>();
     private final ExtremityMonitor xExtremityMonitor = new ExtremityMonitor();
+    private final int backgroundColor;
     private final Paint axisPaint;
     private final Paint xAxisMarkerPaint;
     private final Paint gridPaint;
@@ -306,6 +307,8 @@ public class ChartView extends View {
             }
         });
 
+        backgroundColor = ThemeUtils.getBackgroundColor(context);
+
         float scale = context.getResources().getDisplayMetrics().density;
 
         axisPaint = new Paint();
@@ -538,7 +541,7 @@ public class ChartView extends View {
         synchronized (chartPoints) {
             canvas.save();
 
-            canvas.drawColor(Color.WHITE);
+            canvas.drawColor(backgroundColor);
 
             canvas.save();
 

src/main/java/de/dennisguse/opentracks/util/ThemeUtils.java

@@ -0,0 +1,24 @@
+package de.dennisguse.opentracks.util;
+
+import android.content.Context;
+import android.content.res.Resources;
+import android.util.TypedValue;
+
+import androidx.core.content.ContextCompat;
+
+public class ThemeUtils {
+
+    private ThemeUtils() {
+    }
+
+    /**
+     * Get the material design default background color.
+     */
+    public static int getBackgroundColor(Context context) {
+        TypedValue typedValue = new TypedValue();
+        Resources.Theme theme = context.getTheme();
+        theme.resolveAttribute(android.R.attr.colorBackground, typedValue, true);
+
+        return ContextCompat.getColor(context, typedValue.resourceId);
+    }
+}