hook_user_login not working
Only alter hooks and some specific theme-related hooks can be implemented in a theme.

hook_user_login isn't one of them, you need to implement it in a module.

To create a custom module see the official Creating custom modules docs. You essentially need a .info.yml file and a single .module file containing this:

function MYMODULE_user_login($account) {
  // Your code here
}