Things that can normally be changed are choise of colors, geometry, and fonts. In addition, many programs allow X resources to control specific aspects of their operation. X resources can be formulated to affect all applications, all instances of a particular program, or, possibly, a single instance of a program. This file will be merged in after the normal recources of the environment in use. You may thus make additions or override any resource value.
Commands run at startup or switch of environment.
A few programs, such as mailbox watchers, clocks, load-meters, and a terminal emulator, are normally started when you log in. You may specify a sequence of commands of your own, to be used instead of the standard one. You may thus choose a different set of programs to run, or different locations of windows on screen. It is recommended that you copy the startup file from a standard environment and modify that. To run commands only at login and logout, you may put them in files called .Xlogin and .Xlogout respectively.