GRANT/REVOKE from all tables in database

Question

I've created a database in Postgres, like so:

CREATE DATABASE mydb

I've created a user (role), like so:

CREATE USER myuser

I would now like to GRANT certain permissions (e.g. SELECT) to this user for all tables in this database. How can I achieve this?

The closest thing I can find is the GRANT ... ON ALL TABLES IN SCHEMA someschema, but I don't think this addresses the issue, since a single "database" can have multiple "schema", and I'd like these permissions to apply to all existing tables in all existing schema, as well as all future tables in all future schema.

Similarly, what would then be the equivalent to REVOKE all permissions for all tables in a database?