Add black theme (#726)

* Add black theme

On amoled screens a completely black theme helps save battery,
besides looking awesome!

* Fix first set of merge request issues except the dialog.

* Black theme inherits from dark and only the different values
  are declared
* Make nav bar translucent (for some reason content does not
  go behind it in main activity. Need to investigate)
* Remove nav bar line
* Fix toolbar color

* Fix dialog issue with black theme.

Revert translucent navigation.

* Translations updated, as well as possible from online sources.

* Make login screen respect black theme

app/src/main/res/values-zh-rSG/strings.xml

@@ -160,6 +160,7 @@
     <string-array name="app_theme_names">
         <item>黑夜</item>
         <item>白天</item>
+        <item>黑色</item>
         <item>自动切换</item>
     </string-array>