Kenneth from Stripe here The demo is using https://hyper.is/ with it's standard dark theme and terminal splitting which is built in.
looks like just your regular iterm2 split panes?
alternately, you can use tmux which lets you detach sessions, rotate panes, and configure them in whatever arrangement you'd like! Would highly recommend it :)
Alternatively iTerm2 also comes with a native tmux integration. I personally prefer using tmux on my own but have several friends who prefer the iTerm tmux integration (copy-pasting text is definitely easier).