Can a sub-user use the master account’s custom templates?

Yes. On your master account, when you set up a customized template at the top of the page, set the toggle switch for "Allowed for users" to "On" to make the template applicable for sub-users.
You can also quick-add/remove the application for sub-users from the tables of existing custom templates.

Note that in addition to the templates you make applicable for sub-users, users can also create their own customized templates.