Make help pages work in dark theme

This maintains the black-on-white theme of the HTML views even for the
dark theme.

OpenKeychain/src/main/java/org/sufficientlysecure/keychain/ui/HelpMarkdownFragment.java

@@ -78,6 +78,7 @@ public class HelpMarkdownFragment extends Fragment {
             Log.e(Constants.TAG, "IOException", e);
         }
 
+        text.setBackgroundColor(getResources().getColor(android.R.color.white));
         // no flickering when clicking textview for Android < 4
         text.setTextColor(getResources().getColor(android.R.color.black));