Preview Modes

i’m very new to all this and helping someone create a website! I’ve installed XAMPP, installed wordpress and using the chstomizr theme. It’s been going ok until the other day, but i’m not sure whats happened. When logged in as localhost/websitename/wp-admin…the preview modes for the computer, tablet and mobile are all exactly the same (look like tablet/mobile view)! A few days ago it was working ok and the computer view looked like a computer view..i’m not sure whats gone wrong. When on web as localhost/websitename it looks exactly like the computer view should do, so why in preview mode does it not?