Resource environment provider (REP) custom properties are global in scope, but you can use a consistent naming convention for a theme or module that applies across multiple themes.
For properties that are used only by a single theme, prefix with yourTheme. For those properties that are used only by a single module, prefix with yourModule. Modules do not belong to any particular theme and can be used by any theme. It is a good practice to put your modules in one web app and each theme in its own separate web app.
Replace occurrences in italics with the actual names of your items.