A web-site is laid-out according to the instructions in an HTML file, and styled using the instructions in a CSS file. These formats are what web browsers understand.
It is possible to use C++ on the server-side to create HTML documents and send them to the users, and to do whatever else is necessary upon receipt of a request from a client.
Neither HTML nor CSS are programming languages. But C++ is, and can be used to create programs, or applications, that do whatever the programmer wants.