Add method that tells the app/backend that we are using a dark mode theme

Headers/Additions/GNUstepGUI/GSTheme.h

@@ -783,6 +783,41 @@ APPKIT_EXPORT_CLASS
  */
 - (NSString*) license;
 
+/**
+ * Returns YES if Theme is able to display in Light NSAppearance
+ * For a theme supporting both Light and Dark modes, it should return both
+ * supportsLightAppearance and supportsDarkAppearance
+ * Light is the "default" of the GNUstep theme like Cocoa Aqua.
+ */
+- (BOOL) supportsLightAppearance;
+
+/**
+ * Returns YES if Theme is able to display in Dark NSAppearance
+ * For a theme supporting both Light and Dark modes, it should return both
+ * supportsLightAppearance and supportsDarkAppearance
+ * Light is the default Aqua and GNUstep "classic", but a GNUstep can be dark only.
+ */
+- (BOOL) supportsDarkAppearance;
+
+/**
+ * Returns YES if displaying in Light NSAppearance (or non-Dark)
+ * Should match if theme is equivalent to Cocoa names like
+ * NSAppearanceNameAqua or NSAppearanceNameAccessibilityHighContrastAqua
+ *
+ * Being only Light and Dark supported, must be opposite of inDarkAppearance
+ */
+- (BOOL) inLightAppearance;
+
+/**
+ * Returns YES if displaying in dark NSAppearance
+ * Should match if theme is equivalent to Cocoa names like
+ * NSAppearanceNameDarkAqua or NSAppearanceNameAccessibilityHighContrastDarkAqua
+ *
+ * Being only Light and Dark supported, must be opposite of inLightAppearance
+ */
+- (BOOL) inDarkAppearance;
+
+
 @end
 
 /**

Source/GSTheme.m

@@ -1339,6 +1339,69 @@ - (NSString *) license
   return [[self infoDictionary] objectForKey: @"GSThemeLicense"];
 }
 
+- (BOOL) supportsLightAppearance
+{
+  NSString *val;
+  BOOL r;
+
+  r = YES; // default themes are light
+  val = [[self infoDictionary] objectForKey: @"GSThemeSupportsLightAppearance"];
+
+  if (val && [val isEqualToString: @"YES"])
+    r =  YES;
+
+  return r;
+}
+
+- (BOOL) supportsDarkAppearance
+{
+  NSString *val;
+  BOOL r;
+
+  r = NO; // default themes are light
+  val = [[self infoDictionary] objectForKey: @"GSThemeSupportsDarkAppearance"];
+
+  // Do we have the key or is it an old GSTheme?
+  if (val == nil)
+    {
+      // try heuristic on the theme name
+      if ([[self name] hasSuffix:@"Dark"])
+        {
+          r = YES;
+        }
+    }
+  else
+    {
+      if ([val isEqualToString: @"YES"])
+        {
+          r =  YES;
+        }
+    }
+
+  return r;
+}
+
+- (BOOL) inLightAppearance
+{
+  // NOTE: maybe we should cache this in an ivar
+  if (![self supportsLightAppearance] && [self supportsDarkAppearance])
+    {
+      return NO;
+    }
+  return YES;
+}
+
+- (BOOL) inDarkAppearance;
+{
+  // NOTE: maybe we should cache this in an ivar
+  if (![self supportsLightAppearance] && [self supportsDarkAppearance])
+    {
+      return YES;
+    }
+  return NO;
+}
+
+
 @end
 
 @implementation	GSTheme (Private)