If you have some CSS skills, it would be possible with the BuddyPress Login widget but it requires a little bit of CSS styling.
Otherwise, if you are using the Login With Ajax plugin (https://wordpress.org/plugins/login-with-ajax/) an alternative would be to use the Login With Ajax widget and choose the “modal” template. That will give you a “Login” button that opens a modal with a login form.
You're trying to setup your theme but you're experiencing errors when importing the demo content? Or you've just followed the setup guide but your website doesn't look exactly like our demo? These are common issues for which you can find easy and quick fixes.