Issue with installing a new theme..PLEASE HELP!!! ASAP

I am trying to install a theme and I know you to to Appearance > Theme > then choose a file and install it. But everything on the page is completely cut off. All the way from the top of the menu and all the way down. So even if I choose the file I am trying to install I don’t see the button to click on to actually install. Has anyone had this issue before. Hope this makes sense…Need help ASAP