Add dark app theme support

Author: MHShetty

app/src/main/java/org/grapheneos/pdfviewer/PdfViewer.java

@@ -158,6 +158,7 @@ protected void onCreate(Bundle savedInstanceState) {
         setContentView(R.layout.webview);
 
         mWebView = findViewById(R.id.webview);
+        mWebView.setBackgroundColor(Color.TRANSPARENT);
 
         if (BuildConfig.DEBUG) {
             WebView.setWebContentsDebuggingEnabled(true);

app/src/main/res/values-night/styles.xml

@@ -0,0 +1,9 @@
+<resources>
+
+    <!-- Base application theme. -->
+    <style name="AppTheme" parent="Theme.MaterialComponents.DayNight.DarkActionBar">
+        <!-- Customize your theme here. -->
+        <item name="android:windowBackground">#000000</item>
+    </style>
+
+</resources>

app/src/main/res/values/styles.xml

@@ -1,7 +1,7 @@
 <resources>
 
     <!-- Base application theme. -->
-    <style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
+    <style name="AppTheme" parent="Theme.MaterialComponents.DayNight.DarkActionBar">
         <!-- Customize your theme here. -->
     </style>