This module allows you to specify the view mode of Field collection entities on the edit form of the entity itself. Fiel collections usually only have a single view mode ('full'). With this module whatever you choose to have as an allowed value of the field_collection_view_mode field, becomes a view mode of field collections. You can then configure (or theme) these view modes as usual, and on the edit form of each field collection, you can now choose your view mode.
To give an example, this module was created for a custom Paragraph module which allowed to add multiple paragraphs to a node for instances. Each paragraph had a Heading, a content field, and an image field. For each paragraph you could choose whether the image would be inset on the right or the left of the text or whether it was to be displayed in full-width.
Dependencies
Development
This currently a sandbox project, because I'm not sure whether it is generally useful enough to be a full module. If you use this module or would like to, please open an issue and explain your use-case and/or explain why you think this should/should not be promoted to a full module! :)