Download the screensaver:

Windows 10/8/7
Unzip, then doubleclick AlysTheThief-pc.msi to install. To change any preferences for your screensaver, go to your Display properties in the Control Panel.

Mac OSX Mojave and higher
Unzip, then doubleclick the saver file to install.

Mac OSX High Sierra and earlier
Open file after download completes and it will automatically install itself as a screensaver.