Make one blueprint actor which we can call the ‘theme controller’. All it contains is an integer, which denotes the theme to use.
Then in your building actor class, you have a run in editor event like this:
Meshes and materials are just arrays of meshes and materials ( of course ). This way, you just set the theme in one place, then select all the actors on press the ‘read theme’ button once.
You can do it so that they will find the controller and you can just press a button on the controller to send out a new theme, but the game needs to be running for that.