Update HTMLViewer to support dark theme.

Fixes: 136033465
Test: visual inspection
Change-Id: I7231ed7cd659ac594c316eb884aa5caa72bfea0b
diff --git a/AndroidManifest.xml b/AndroidManifest.xml
index 660c625..655127b 100644
--- a/AndroidManifest.xml
+++ b/AndroidManifest.xml
@@ -25,7 +25,7 @@
             android:supportsRtl="true">
         <activity android:name="HTMLViewerActivity"
                 android:label="@string/app_label"
-                android:theme="@android:style/Theme.DeviceDefault">
+                android:theme="@android:style/Theme.DeviceDefault.Settings">
             <intent-filter>
                 <category android:name="android.intent.category.DEFAULT" />
                 <action android:name="android.intent.action.VIEW" />