Added admin theme class and css for claro theme
As of #8 i see, that we need here a switch to apply the changes only for claro theme.
So i applied a custom css class to the detail wrapper of the forms element, to determine the admin theme context. The class is
admin-theme-[THEMENAME]
This way we can add css for different admin themes separately. Also i added the css from #6 wrapped in the 'admin-theme-claro' theme class, so it only will be applied for claro.