Diagram editor: The theme doesn't change when the workflow contains errors or is empty

[handreyrc]

Currently the canvas needs to be reloaded in order to apply a different theme. However, if the diagram has errors or is empty it is not reloaded. In this scenario the error page or last valid diagram rendered is kept on screen util there is a valid workflow to be rendered.

Notice that this behavior is what allows the user to still see and navigate through the diagram even if it has errors.

The goal of this task is to find a way to improve the user experience by applying the theme in these scenarios.