Ghost is a great blog engine but it misses a built-in searching capability. Trying to find some help on the Ghost forum I finally found these resources:

https://www.hauntedthemes.com/ghost-search-our-first-open-source-library-for-ghost

https://github.com/HauntedThemes/ghost-search

After few tries to understand how it works, I finally have a functional Casper-based theme for my blog with a great search functionality.