Simply put:
Evars can be defined anywhere in the theme
Themevars.rc
Theme.rc
Where ever they can be neatly put.
If I define "apps C:\litestep\themes\mytheme\apps" in any of the files you place it in, it can be called from a bang like this one.
*Popup "apps" !DynamicFolder:"$apps$"
or
*Popup "apps" !Execute ["$FileManager$" /e, "$apps$"]
or
*Shortcut "" -10 0 image.png .none none #0 !Exec "$apps$"

Also, spaces in the path will cause issues.
Use the dos name if needed
C:\progra~1\apps