Does the 18.04 Final Freeze (released yesterday?) have the Suru icon pack? I haven't tried downloading it yet.
Currently Ambiance (and Humanity / Ubuntu Mono icons) is still default in Ubuntu 18.04. The Communitheme (which Suru is a part of) is available to install as a snap from the Ubuntu Software application.
Install Communitheme snap via Ubuntu Software or using the command line:
snap install communitheme
Next, logout (a reboot might be needed, I'm not 100% sure) and select the “Communitheme snap session” from the login screen.