Can a sub-account 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 sub-accounts" to "On" to make the template applicable for sub-accounts.
You can also quick-add/remove the application for sub-accounts from the tables of existing custom templates.

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