Add option for AMOLED night mode

app/src/main/java/org/isoron/uhabits/helpers/UIHelper.java

@@ -202,8 +202,17 @@ public abstract class UIHelper
         switch(getCurrentTheme())
         {
             case THEME_DARK:
-                activity.setTheme(R.style.AppBaseThemeDark);
+            {
+                SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(activity);
+                boolean pureBlackEnabled = prefs.getBoolean("pref_pure_black", false);
+
+                if(pureBlackEnabled)
+                    activity.setTheme(R.style.AppBaseThemeDark_PureBlack);
+                else
+                    activity.setTheme(R.style.AppBaseThemeDark);
+
                 break;
+            }
 
             case THEME_LIGHT:
             default:

app/src/main/res/values/strings.xml

@@ -157,4 +157,7 @@
     <string name="troubleshooting">Troubleshooting</string>
     <string name="help_translate">Help translate this app</string>
     <string name="night_mode">Night mode</string>
+    <string name="use_pure_black">Pure black for night mode</string>
+    <string name="pure_black_description">Use pure black instead of grey backgrounds in night mode . Reduces battery usage in phones with AMOLED display.</string>
+    <string name="interface_preferences">Interface</string>
 </resources>

app/src/main/res/xml/preferences.xml

@@ -21,8 +21,8 @@
 <PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android">
 
     <PreferenceCategory
-        android:key="behaviorCategory"
+        android:key="interfaceCategory"
-        android:title="@string/behavior">
+        android:title="@string/interface_preferences">
 
         <CheckBoxPreference
             android:defaultValue="false"
@@ -30,6 +30,18 @@
             android:summary="@string/pref_toggle_description"
             android:title="@string/pref_toggle_title"/>
 
+        <CheckBoxPreference
+            android:defaultValue="false"
+            android:key="pref_pure_black"
+            android:summary="@string/pure_black_description"
+            android:title="@string/use_pure_black"/>
+
+    </PreferenceCategory>
+
+    <PreferenceCategory
+        android:key="reminderCategory"
+        android:title="@string/reminder">
+
         <ListPreference
             android:defaultValue="@string/snooze_interval_default"
             android:dialogTitle="@string/snooze_interval"