Regretfully, there is no feature in X that will make the Navbar's background change to white when it hits a certain element. You will need a custom script for that which we could not create as part of our support service. It's also not recommended since you're not a developer.
Overall though, this requires some setting up unlike other ready to use themes. That is because, X and Pro provides flexibility. That is why you need to actually build. Other themes are static. They only do one thing. I believe a full height option is available in other builders. But, it does not have the flexibility of Cornerstone or Pro's Content Builder so there's a trade-off between flexibility and ease of setup.
There is currently no plans for this type of design in X. This is achievable in Pro without custom coding. I checked your sample site and I could replicate it with Pro. Not the exact copy but close. But, if you're looking for a no setup method though, regretfully, that is not possible.
Thank you for understanding.