chore: add a login class to mgt-login styling story (#2586)

Signed-off-by: Martin Musale <[email protected]>

Author: musale

stories/components/login/login.styles.js

@@ -15,7 +15,7 @@ export default {
 };
 
 export const customCSSProperties = () => html`
-<mgt-login></mgt-login>
+<mgt-login class="login"></mgt-login>
 <style>
   .login {
     --login-signed-out-button-background: red;