Use addThemeVariants in documentation

[Peppe]

Vaadin 12 brought us the handy componentaddThemeVariants(String) method for components. However in the documentation there is still usage of button.getElement().setAttribute("theme", "contrast primary"); at least in https://vaadin.com/docs/v12/flow/theme/using-component-themes.html. Update examples across the board to use the new API .

[DiegoSanzVi]

There was already a commit about it: "Add theme variants documentation (#216)"

In the documentation, he mentioned the old way as an alternative,

"If you want more flexibility (which may come in handy when adding non-standard theme variants to the component), you can manipulate the value of the theme attribute directly:"

b214e41#diff-268e59436474fb538c76d41f7c8715f8

Should this issue be closed?

[ZheSun88]

As Diego said, This ticket has been completed. closed.