How to respect copyright for design and functionality?

Question

Copyright

I am offered to buy a script for social. network, written by myself. Out of the box design - a copy of a well-known Russian chain 5-10 years ago. I read here that the functionality of the program is not subject to copyright law. And what about the design?
Let's do our own design.
Where is the line in design: one's own or someone else's?