Slider: use the _classes method. Fixes #7053: Ability to customize style-related CSS classes
Spinner: use the _classes method. Fixes #7053: Ability to customize style-related CSS classes
Progressbar: use the _classes method. Fixes #7053: Ability to customize style-related CSS classes
Menu: use the _classes method. Fixes #7053: Ability to customize style-related CSS classes
Button: use the _classes method. Fixes #7053: Ability to customize style-related CSS classes