When we write our applications, it is up to us, the programmer to choose the look and feel of the program for ourselves. This was a deliberate design choice made by MIT and is one reason why X Window System has become a standard (vendors would refuse to use a standard if it forced you to use the enemies interface).
In a commercial setting you will probably choose either to
follow the
Open Look style - on Sun workstations or
Motif - on most other vendors workstations
CDE - Common Desktop Environment.
Xlib - the low level X routines
Xt - the Intrinsics Toolkit
Xaw - the Athena Widgets and
Xm - the Motif widgets