Nothing of the sort 
In the UI goto: Administration->Layout
When running a released copy of Rocket.Chat consider it almost like an .exe you don’t go poking around inside of it, because you will break something. It is compiled down.
Instead you use the ways we provide to edit layout.
Much different than typical php project deployed via cpanel or any other website that you drop your assets in a folder and you edit away.
Even though you have checked out the code from git, when you run docker-compose you are actually running a released copy of Rocket.Chat not the raw source code from the directory you are in. But instead it goes out and fetches our latest release(at this time rocketchat/rocket.chat:0.61.1)