fix dark theme preference for dev version: use different preference name

Signed-off-by: tobiasKaminsky <tobias@kaminsky.me>

src/main/java/com/owncloud/android/ui/activity/SettingsActivity.java

@@ -695,7 +695,7 @@ public class SettingsActivity extends ThemedPreferenceActivity
 
         loadStoragePath();
 
-        ListPreference themePref = (ListPreference) findPreference("darkTheme");
+        ListPreference themePref = (ListPreference) findPreference("darkMode");
 
         List<String> themeEntries = new ArrayList<>(3);
         themeEntries.add(getString(R.string.prefs_value_theme_light));

src/main/res/xml/preferences.xml

@@ -28,7 +28,7 @@
             android:key="storage_path"/>
         <ListPreference
             android:title="@string/prefs_theme_title"
-            android:key="darkTheme"
+            android:key="darkMode"
             android:summary="%s" />
 	</PreferenceCategory>
 	<PreferenceCategory