Thank you for asking.
> How to change the client section background color to grey as in demo
Go to Edit row > Row Styles > Theme > Background Color https://cloudup.com/cwcU41E7tHg
> … what is the name of widgets which are showing in demo after “our services” and before “our team” and after “our team”
Those are built using the Layout Builder. There are Text and Sydney FP: Call to action widgets inside it. https://cloudup.com/clqWrQoJpPD
> … and how to set that links which scroll them below to sections.
In the “Link for the button” field of Sydney FP: Call to action widget setting, enter the element target ID which is preceded with a
#. To find the element ID, use the browser’s inspector tool https://cloudup.com/cFTDj1wGUvJ
To familiarize yourself with our demo setup, I’d suggest you to do importing the settings file and demo content; follow this quick start guide.