Fix true black theme in all activities

Closes #32. Also tested on API 19.

app/src/main/res/layout/activity_auth.xml

@@ -5,6 +5,7 @@
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     android:fitsSystemWindows="true"
+    android:background="?attr/background"
     tools:context="com.beemdevelopment.aegis.com.impy.aegis.ui.AuthActivity">
     <LinearLayout
         android:orientation="vertical"

app/src/main/res/layout/activity_groups.xml

@@ -5,6 +5,7 @@
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     android:fitsSystemWindows="true"
+    android:background="?attr/background"
     tools:context="com.beemdevelopment.aegis.com.impy.aegis.ui.GroupManagerActivity">
 
     <androidx.core.widget.NestedScrollView

app/src/main/res/layout/activity_slots.xml

@@ -5,6 +5,8 @@
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     android:fitsSystemWindows="true"
+    android:background="?attr/background"
+
     tools:context="com.beemdevelopment.aegis.com.impy.aegis.ui.SlotManagerActivity">