However, I couldn’t find any similar theme from the theme market. Is there any way to know what theme they are using?
Many of the websites are using a customized version of the Casper theme. Many are custom designed and not available as public themes, for example, Buffer and Duolingo. Some are using my Nubia theme, for example, The Gradient, and Harvard International Review.
Related: I think you will find the Ghost Inspire website helpful. Currently includes 100+ Ghost website examples with the used theme.