#1542 Adding gantt to showcases

docs/theming.md

@@ -143,6 +143,35 @@ When adjusting a theme it might be helpful to look at the theme with these diagr
             end
 ```
 
+### class diagram
+
+```mermaid
+%%{init: {'securityLevel': 'loose', 'theme':'base'}}%%
+
+classDiagram
+	Animal "1" <|-- Duck
+	Animal <|-- Fish
+	Animal <--o Zebra
+	Animal : +int age
+	Animal : +String gender
+	Animal: +isMammal()
+	Animal: +mate()
+	class Duck{
+		+String beakColor
+		+swim()
+		+quack()
+	}
+	class Fish{
+		-int sizeInFeet
+		-canEat()
+	}
+	class Zebra{
+		+bool is_wild
+		+run()
+	}
+
+```
+
 ### Gantt
 
 ```mermaid