Installing Source Theme

Hi there, I’m following the youtube video by Ghost (https://www.youtube.com/watch?v=Tbelo_YoB4c&t=308s) and have installed and Activated Source as my theme, but Casper is stubborn and won’t go away. I’m follow the first minute and a half of the video (although the Advanced button is no longer present, I found where to Activate anyway), but no dice. Help? Thanks in advance, Marty