Add theme setting

This prepares that different themes can be added. Currently light and
dark will just use the default OK theme.

OpenKeychain/src/main/res/values/arrays.xml

@@ -61,4 +61,12 @@
         <item>@string/key_size_custom</item>
     </string-array>
 
+    <string-array name="theme_entries" translatable="false">
+        <item>@string/theme_dark</item>
+        <item>@string/theme_light</item>
+    </string-array>
+    <string-array name="theme_values" translatable="false">
+        <item>"dark"</item>
+        <item>"ligh"</item>
+    </string-array>
 </resources>