Spotfire files can import Custom Themes from other Spotfire Files or Spotfire Files in the Library. However, that is a one time adoption of the Custom Theme settings. If the theme from the originating file changes, other files "borrowing" that theme do not get changed as well.
This is important when a company has Custom Themes that are used across the organization. New Spotfire Files can import the Custom Theme, but are not updated when the template for the Custom Theme is changed.
I think there should be an option for Spotfire Files, which import Custom Themes from the Library, to be linked to their themes..