The Berlin theme is a Magic Pages exclusive Ghost CMS theme. If you're hosting your website with Magic Pages, installing it is straightforward and simple.
Log into your Ghost website's admin panel and go to Settings. Look for "Design & Branding". You should now see a gallery of different themes.
Simply select the Berlin theme and install it.
Additionally, you might want to edit your routes.yaml to add the Berlin-specific routes. You can find more on that here.