Add theme support for post thumbnail isn’t working from theme class

If that’s the whole class then you don’t have a method called get_instance

By the way, even if you had a get_instance method, you are calling get_instance as a static method, __construct gets called only when creating a new instance from the class using the new keyword, like this

$obj = new WCS_THEME();

That’s basically it, Going the your class in it’s current state, all you need to do is a new instance, as shown in the code above.

Don’t forget about the namespace we creating the instance