Adding Aditional Classes to a Wordpress Block
I think that this would be just a step too far for the admin of the Village Hall!
The wordpress.com website says"

Custom CSS - the VH seems to have trouble with the basics!
Explorer Price Plan
£7 per month billed annually (£84).

Like so many other features of Wordpress extra features are only available on more expensive subscriptions.
I guess this makes sense, the "novice" website creator will not know what a "class" is, but there again if they do then they probably shouln't be using Wordpress.
Explorer, Creator, and Entrepreneur plans seem to be new to Wordpress
Custom Classes
The wordpress page "Add Custom CSS" supposedly explians how:

Wordpress have made this doubly confusing as there seems to be a difference between adding Custom Classes to the basic editor and to the Block Editor.