I would like to change the code for the popup that appears in the top right when you are not signed in and try to make a post. Instead of sending you to the login url, the theme I am using (Newspaper 9) has a login/register popup that I would like to use instead.
I'm sorry, but if you use only pop-up login/registration you'll need some custom js code. wpForo doesn't have such functionality.
If you already have a page with custom login/registration form provided by other profile plugins you can insert this page URL in according authorization field of Forums > Settings > Members admin page. This replaces wpForo login/registration page with the custom page.
More info here: https://wpforo.com/docs/root/wpforo-settings/members-settings/#custom-authorization