feat(theme): add forSystemNightMode helper on AwfulTheme (#534)

Awful.apk/src/main/java/com/ferg/awfulapp/provider/AwfulTheme.java

@@ -136,6 +136,34 @@ public static String getCustomThemePath() {
     }
 
 
+    /**
+     * Issue #534: pick the night-mode counterpart of the user-selected
+     * light theme when the system reports it is currently night.
+     *
+     * Returns the supplied light theme unchanged when the system is in
+     * day mode or in the unspecified state. Mapping is conservative,
+     * only the standard app themes are translated. Custom themes are
+     * returned as-is so users keep their explicit choice.
+     */
+    @NonNull
+    public static AwfulTheme forSystemNightMode(@NonNull Context context, @NonNull AwfulTheme lightTheme) {
+        int mode = context.getResources().getConfiguration().uiMode
+                & Configuration.UI_MODE_NIGHT_MASK;
+        if (mode != Configuration.UI_MODE_NIGHT_YES) {
+            return lightTheme;
+        }
+        switch (lightTheme) {
+            case DEFAULT:
+            case CLASSIC:
+                return DARK;
+            case CUSTOM_DEFAULT:
+                return CUSTOM_DARK;
+            default:
+                return lightTheme;
+        }
+    }
+
+
     /**
      * Get a forum's specific theme, if it has one.
      *