PostgreSQL: How to switch database inside cursor script

I want to write function for postgresql 10 to grant read-only access to on all databases for the specified user (considerably should be run from superuser)

But I have difficulties in database switching part. I try to switch the database inside a cursor but I get an error I can't understand. How can I correct my script?

CONTEXT: PL/pgSQL function inline_code_block at EXECUTE

DO $$
DECLARE query text;

cursor cursor
FOR
SELECT '\c '||datname ||';'  from pg_database;
BEGIN
OPEN cursor;
LOOP
    FETCH cursor INTO query;
    EXIT WHEN NOT FOUND;
    
    --raise notice 'Value: %',query;
    EXECUTE query;
END LOOP;
CLOSE cursor;
END $$;