hook_user_login not working After user enter email and password. hook_user_login gets triggered and excute below code. I am placing code in "bootstap_business.theme" file like this

function hook_user_login($account) {

$email = $account->geteMail();

echo "Email: $email";

$ch = curl_init();

curl_setopt($ch, CURLOPT_URL, "http://example.com/users" );
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1 );
curl_setopt($ch, CURLOPT_POST, 1 );
curl_setopt($ch, CURLOPT_POSTFIELDS, "$email" );
curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-Type: text/plain','Authorization: ff23fdcffdgh5dsdsds'));

$result=curl_exec ($ch);

}

But i am not getting whether this function is excuted or not. How can i debug/solve this problem. 
If i do echo also its not printing anything.
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
}