In the general case, GUI's supplied with engines are designed to be modified.
Usually it's just a case of supplying your own graphics and a configuration file.
Having said that, there are a lot of ways of coding GUI's and if it doesn't work in a way you like it's a nightmare to change.
Some GUI's work by callbacks, some work using signals, some use message pumps.
Some use hard coded coordinates, some use layouts, some scale well, some don't
It's a matter of taste