Note: conditionals are currently only usable in the newest builds of LiteStep. I recommend 11-28-2000, the current stable build.

Conditionals are sub-scripts that allow the themer (you) to have different step.rc elements loaded depending on certain variables. The most common method is for a resolution dependent conditional:
If ResolutionX = 640 or ResolutionY = 480
  *Shortcut "" -36 117 BarRightBlank.bmp  .none .none #1200s  !none
ElseIf ResolutionX = 800 or ResolutionY = 600
  *Shortcut "" -36 117 BarRightBlank.bmp  .none .none #1200s  !none
  *Shortcut "" -36 717 BarRightBlank.bmp  .none .none #1200s  !none

This works because the variables ResolutionX and ResolutionY are internal variables, and are automatically set by LiteStep on load.

Another way, using internal variables, would be a conditional using the UserName variable to select different components, so that a themer can have specific settings for himself, and generic settings for all other users:
If UserName = Maur
  ShutdownWindows c:\windows\shutdown.exe
  RebootWindows   c:\windows\reboot.exe
  RestartWindows  c:\windows\restart.exe
  ShutdownWindows "RUNDLL32.EXE shell32.dll,SHExitWindowsEx 0x1"
  RebootWindows   "RUNDLL32.EXE shell32.dll,SHExitWindowsEx 0x2"
  RestartWindows  "RUNDLL32.EXE shell32.dll,SHExitWindowsEx 0x3"

A third major way would be to have a numeric set, and a variable that is defined by the themer, in this case:
popupimgstyle 1

If popupimgstyle = 1
  PopupDir ..\popupsEN\
    popupstyle_en       "• Entransi popups"
    popupstyle_en2      "Entransi2 popups"
ElseIf popupimgstyle = 2
  PopupDir ..\popupsEN2\
    popupstyle_en       "Entransi popups"
    popupstyle_en2      "• Entransi2 popups"

And then have a selection entry in the step.rc's popup section which changed the variable $popupimgstyle$ according to the user's selection:

    *Popup "Popup Settings..." Folder
      *Popup "$popupstyle_en$" !execute !rc-set popupimgstyle "1"!recyclels
      *Popup "$popupstyle_en2$" !execute !rc-set popupimgstyle "2"[!recyclels]
    *Popup ~Folder

This way requires lsrc.dll to be loaded, in order for the !rc-set command to be able to change the variable in the step.rc.

Of course, as a themer you'll certainly come up with many more ideas, depending on how your theme is designed (eg: possibly a combination of conditionals and LSBox, to have different theme layouts for different user styles).

All examples taken with permission from BarQNX by Maur